Ash is so stupid.

I agree for totally different reasons.
But Ash is... well, Ash.
He keeps "resetting" at the start of every region.

The motto is "Gotta Catch 'Em All" not "Gotta use one overpowered midget rat who can be stronger than Arceus"

Am I not correct? I mean, he barely catches any Pokemon. He also got all the Kanto starters and Pikachu, but only evolved Charmander.

As of the most recent episode, Ash has a Pikachu, Charizard, Unfezant, Oshawott, Pignite, Snivy, Scraggy, Leavanny, Palpitoad, Boldore, Krookodile, Bulbasaur, Kingler, Muk, Tauros (30 of them), Snorlax, Heracross, Bayleef, Quilava, Totodile, Noctowl (shiny), Donphan, Swellow, Sceptile, Corphish, Torkoal, Glalie, Staraptor, Torterra, Infernape, Buizel and a Gible. So far I know, Squirtle and Primeape are in training, and Gliscor's status is unknown.
He has the most Pokémon of all the main anime characters, as far as I know.

Team rocket is also quite stupid. Why steal Ash's Pikachu when there's millions more out there?

Probably because Ash's Pikachu was able to cause massive destruction in the second episode? That's my guess.