Because Iconography and Pokemon Let's Go
Personally I don't think too much stock should be put into Let's Go, it's kinda its own thing. When I think of Red, I don't think of Machamp, but rather more special Pokemon. Do note that, with the exception of Pikachu, Red in the games always had special Pokemon, besides in Let's Go:
the three starters
Lapras (gift Pokemon)
Espeon (gift Pokemon)
Snorlax (sort of like an unavoidable boss fight)
Machamp and Arcanine just kind of seem unfitting for him, at least I think so.
Personally I'd give Red and Blue those teams:
Red:
Main: Pikachu, Charizard, Espeon
Support: Lapras, Snorlax, shiny Aerodactyl (as a nod to the Manga, as well as still being special on account of being a fossil)
Blue:
Main: Blastoise, Alakazam, Exeggcutor
Support: Pidgeot, Arcanine, Machamp (could also fit as a main fighter) or Eevee (to go with Red's Pikachu and as a reference to the Anime and Yellow)
As for why I switched some of those around, I feel like Arcanine is much more iconic on Blue, given he's the Charizard replacement for his team, same deal with Exeggcutor, who was missing entirely and those two still pop up regularly. Meanwhile Gyarados seems redundant due to having Blastoise. And compared to Red, Blue has used Machamp much more than him, so I feel it's a better fit.
And just for fun, Prof. Oak:
Main: Mew, Tauros, Dragonite
Sup: Fearow, Nidorino, Rotom
Honestly, Oak's something of a wild card, due to mostly being associated with the starters and the various media all give him different teams. The Manga had him as a mostly Normal-type trainer with Kangaskhan, Fearow, Stantler, etc, while the Anime showed him with Pidgey, Dragonite and, in the 4th movie, Charmeleon. He also caught a Rotom.
Might as well do Prof. Birch:
Instant team, just add babies
that was easy
Just my 2 cents, I spent way too long on this already. I'd main Leaf anyway, if only due to Venusaur alone.