Because humans were/are pokemon in on themselves.

And because pokemon have lived alongside humans since humans were a thing, so there never was a competition. Human has no predator.

To be fair most pokemon evolve through the help of human assistance and not only that but in the real world cats for example evolved by themselves so they could take life easy and live along side humans

Humans used to be Pokémon and they evo

Intelligence, basically the ability to know how to make walls and wepons before pokeballs.
The pokemon company covers this in the series (believe its called) "hissue in the snow"