More combo extender assists!
I cant tell for sure but it kind of looks like youre looking at the system on a very shallow level, like the only combos you see are wolverine, sentinel, wesker and doom.
like, magic series - air magic series - otg/assist - relaunch magic series - otg to super. that what youre describing and thats essentially... the most basic combo and the only characters that function that way for their good combos are the characters I stated. everyone else has jump loops, fly combos, non-super jump combos, bounce combos. the extended combos in that game are very indepth. like, go watch a Dante, Morrigan, Jill, Hulk, or Chris combo video. the stuff you can do is highly varied.
as far as your point about why didnt they just make characters be able to do everything themselves, first of all, they did with doom and thats why hes fkng stupid. second of all that would take all of the variety out of the game.
If everyone could do everything Doom does, the game would be lame and boring because even it would get stale.
But instead you have characters like Spencer, who is absurdly strong, and is guaranteed to kill anyone who gets hit by him. But he needs a strong projectile assist to get in or else his approaches get blown up for free. but which projectile assist you give him depends on what you want to do and who you like, you can give him doom, deadpool, iron man, sent, chris... and then each one of those characters affects how you play your DHCs, your TACs, your Team Supers, how you use your X-factor, who you pick as a third, and so on. the system is a lot deeper than you give it credit for.
But then lets take a character like Viewtiful Joe (my main), unless hes fighting against a devoted keep away team, he doesnt have a problem with projectiles, his is good and his mobility is solid. but he has stubby limbs so sometimes he gets out spaced and cant open people up, he needs a lock down assist to keep people stationary for a second to run mix ups. something like chun lis legs or franks shopping cart or ryu/akumas tatsu. Alternatively he can focus on upping his damage and run a combo extender OTG like wesker since he really doesnt have one of his own
And then you have someone like trish, who is a devoted run away/keep away, she is good at avoiding people but she cant do it for ever, so she needs an (invincible) GTFO assist to protect her. Examples would be haggar lariat, Hulks up charge, dantes jam session. and then each one of those comes with different ways to implement it for combos, and DHCs and so on.
That leads to highly varied teams at the end of the day, because even the same character can play vastly different based on the assist behind them. A V Joe with Ammys cold star is going to attack a matchup very differently then someone that only has Morrigans meter assist giving him bar.
Even then it only makes characters with good assists a necessity. Characters with crappy assists are totally useless unless they are toptier by themselves.
you say this as if its a bad thing.
some characters are bad in the game but have amazing assists (Hsien-Ko, pre-patch Phoenix Wright, Rocket Raccoon, Iron Man)
Some characters are near broken on point, but have unusable assists (Wolverine, C. Viper, Zero)
Some characters are solid in both (Dante, Ammy, Deadpool)
Some characters are made to be on point (Wolverine, Firebrand, Tron Bonne)
Some characters are perfect in second spot (Morrigan, Storm)
Some are designed to be anchors (Phoenix, Strider)
And some have abilities that let them do well in any position (Virgil, Doom, X-23.
finding new and interesting ways to mix and match and combined all of these aspects is what makes the game great.
However, sometimes this is overlooked because the top few characters have too many of the best characteristics (or just TOO good at a few), and not enough of the weaknesses. Doom has amazing rushdown, near uncatchable run away, damaging (for combos) AND safe (for utility) DHCs and the best assist in the game.
Zero can run some of the hardest to block mixups in the game, and then he gets to kill you meter positive and then do the same to your next character this is a damage problem, not a Zero problem)
Virgil just has too much in the speed/range combo and Spiral Swords give him too much control of the pacing of the match, as well as making his combos too good. (Also a damage problem.
Spencer just does to much damage. (unscaled up grapples are ********)
Wesker is similar to doom in that he has too much of everything (albeit to a lesser degree.
So when it comes to Doom and Wesker and Virgil, they have so much utility, it logically doesnt make sense to not have at least one of them on every team. But outside of the top 5 or so, when the system works as intended. Its actually REALLY well done.