What is the point of the Pokedex? Pokemon and Humans have been living side-by-side for all of known history, are we to believe Oak needs a 10-year-old kid to go out and discover fucking Mankey or some shit?

Almost every Pokemon in existence, outside of specifically legendary Pokemon (though some Pokemon labelled mythical or legendary are owned by trainers) is owned by a Human, is partners with a Human, works for Humans, etc.

The only person "discovering new Pokemon" is you, the player. There is no in-universe explanation for this shit.