The similarities are actually very big.
They both followed the same type of design philosophy by reusing the core type of Digimon to remake the same one in similar ways. In addition to that, the Pika clones were consistently shown off in the anime and/or had a mascot status. It's hardly a coincidence. The coincidence is that it's akin to to Ash getting a Starter every region. I guess you could compare Veemon to Raichu instead to a degree. Since Baby Digimon do exist. But Adventure 1 and 2 are kind of weird in the same say Kanto and Johto are, fully connected yet still attempts to be unique and different. All of the core protagonist Digimon partners were intentionally reptilian and literally only Veemon stands out slightly by not being outright related to Fire, but more Wind. And they still have similar levels too. Even Frontier alone uses a similar system. Fusion slightly changes it up, mostly cause the Greymon and Agumon variants are separated. Shoutmon is pretty clear the normal Agumon expy, but Greymon, MetalGreymon, and ZekeGreymon are blatantly ones for, well, the first two are obvious, and the last one is a WarGreymon expy. Shoutmon DX is more of the Omnimon of the group. Technically speaking now you have Omnimon Merciful Mode to a degree with the final Fusion, X7 Superior Mode, but it was more regular X7 that was effectively Imperialdramon Paladin Mode at that point power-wise. Something beyond that didn't realllllly exist yet. Veemon's line is also a reptilian similarity to the normal Agumon line. From a larger variant of the Dinosaur, to a more metallic variant, and finally a more dragon-like variant. Fighter Mode is pretty much meant to mirror Omnimon in terms of being an upgrade, as well as it required a DNA to some degree. Guilmon's entirely line resembles Agumon's except Gallantmon, but it still somewhat does(the armored draconic knight that came from overall strength of character). Frontier's kind of unique in that Flaremon is really the Agumon, despite Agunimon's name. Agunimon takes Greymon's role by being the first core evolution, and is Adult-Class. BurningGreymon is more metallic, just like MetalGreymon, and is Ultimate-Class(or Perfect-Class in Japan). Aldamon is pretty much what happens when you fuse both, though to be fair, WarGreymon does resembles the idea of far more metal and dragon combining by design alone. EmperorGreymon in a way resembles Omnimon, using the power of multiple forms to create one, but not just by your own forms alone. In terms of strength, Imperialdramon Paladin Mode is meant to match Susanomon. Then you have Data Squad, where the Agumon line is pretty much just a remake directly of the normal Agumon line. Though Burst Mode doesn't resemble anything in particular. That's completely new.
The key thing to also remember is that the general rodent remake in Pokemon is that they aren't just rodents, they're also Electric types and resemble a member of the family to some degree, or play a similar role in a way. Pikachu directly didn't get a remake, but Plusle & Minun literally were designed to resemble Pichu. Dedenne couldn't look more like Raichu if you tried. They tried to remake pieces of the family as is. Oftentimes the Pokemon even having issues with their abilities(Pachirisu can't control its electricity in the anime, an issue Pichu has as a core gimmick alone. Since Ash doesn't catch another electric rodent, most of the time that was given to another member of his friend group. Pichu is kind of the only one that doesn't fit it(but note what I said about Kanto and Johto being weird. They were at one point part of the same game in itself, not developed separately. So the anime can't do a lot to try and be similar to the games. Pichu was oddly promoted mainly through movies, not through the regular anime). Emolga is yet another one... who heavily resembles Pachirisu, but a flying variant. People were sure it was an evolution too. Togedamaru is the next electrical rodent from Gen VII, decently resembling classic Pikachu's more "fat" design. And then Morpeko for Gen VII, mostly resembling Raichu yet again.
That's a very consistent similarity. Digimon mostly didn't care that much after Xros Wars(including The Hunters Who Leapt Through Time with Gumdramon). Though worth noting the Gumdramon line kind of ends at a MetalGreymon expy via Superior Mode. Superior Mode Snatcher isn't too off from WarGreymon having a lot more metal to it and all, but that's a stretch. ...Literally and figuratively speaking. Though slightly Applimon does kind of resemble the idea, but despite a reptilian partner(Gatchmon), it doesn't actually attempt to mirror the normal Agumon line even slightly. Also, Mega Pokemon are prettttttty clearly the same idea as Burst Mode. Them being called Mega like the Digimon level is the actual coincidence.
Doesn't change they're anime rivals at all. Following pretty much a core partner formula in itself. It isn't till Diamond/Pearl and Frontier that they really really differ from each other.
Nonetheless, there's a lot of similarities in how they handle their expies for future series. Pokemon just doesn't have actual evolutions for the various electrical rodents, however. So it's pretty hard to make a decent rivalry when they went into different directions with the main monster partner's expy style. But they are still done in a similar yet unique path. Pokemon loves to remake certain archtypes. The starters alone are the same point every time, you have the electric rodents, the pseudo-legendaries, regular legendaries(and mythicals), as well as the common monster rodents starting with Rattata. Digimon mostly does this too, but only for the anime. There's always the wolf-like partner combo to build off of the core protagonist. Sometimes it's not a literal wolf Digimon(as shown in Xros Wars, where the Greymon user has a difficult to work with personality but still has a huge heart in the end). You can even compare this to Ash's core rival in each series. They can often be annoying, but ultimately are a good person. It takes longer for some to get there. Paul is perhaps the first time they were an outright jerk, not just a bit of a jerk at times(Gary, mainly). Generally Ash's core rival can be very abrasive as well. The issue is the anime goes all over the place in some ways, besides having the female teammate, and at least one male one(and sometimes the random kid along), not following as strict of a formula in some way. Digimon tries to be slightly more consistent. Data Squad is perhaps the only one where the wolf partnered-character isn't completely abrasive at first either. Gaomon still resembles the Gabumon line(though MachGaogamon and MirageGaogamon don't heavily resemble WereGarurumon and MetalGarurumon that well. Somewhat on Mach due to being more physical than anything. MirageGaogamon only resembles WarGreymon at best. But it still fits the theme, being a very metallic wolf, and very very powerful. Do note what I said about Burst Mode being its own thing. In fact, ZeedGarurumon, which one could argue is kind of a Burst Mode for MetalGarurumon, actually is also an evolution of MachGaogamon, so. It's kind of awkward. Though in a sense the Burst Modes more resemble Omnimon's power level, but not the role. It's kind of close to Fighter Mode, though far more temporary in how long they can last).
The one thing to remember is the Pokemon anime was not just built on "collecting badges". They actually had a good vs evil dynamic as vastly important, including saving the world as somewhat of a concept from Team Rocket forcing them to(and far bigger deals came up with later teams, as they weren't simply thieves, but their actions heavily affected the world. Ruby/Sapphire kind of borked the idea and had it too short to overall feel as world-threatening as they can be, though). Ash being a "Hero" as is is a part of his character. Hell, it's part of Red's character, his origins too. That's his core story, that he also saved the world from Team Rocket, being a Hero to Kanto in general. The anime doesn't always show the Hero storyline of each game too well, but it's there. So there's still a pretty clear dynamic of good VS evil in both of them. If not a core part of each one. Except Orange Island Adventures and Battle Frontier, who are more spin-offs than regular seasons. And the latter still had Team Rocket causing massive trouble.
The biggest thing that's hard different is how the villain system is treated. Almost every core villain except sometimes the starter one tend to actually be evil and usually destroyed in Digimon(to be revived later). But that's due to good vs evil being the main story, with evolving via friendship/other methods being the secondary part. Pokemon's just the reverse, where showing off Pokemon is a lot more important to its story than saving the world from evil.
None of this is that similar when you have other series like Monster Rancher, which only slightly resembles pieces of Pokemon. It also never stayed in the spotlight to be a real rival either way to the fans or otherwise. Dragon Quest Monsters also didn't have an anime to help it, but it also was too different as is. At best it introduced the idea of breeding, which Pokemon use a similar system, and well, a very clear core partner Monster(Slime was basically Pikachu's equivalent. But they didn't change partners or have a real choice like Pokemon and Digimon did, games too). Obviously Digimon's games weren't too similar to Pokemon's, other than switching it up who your starting Monsters were, and keeping a fairly consistent level set/rules generally. With how evolution works and all.
