Hey Pokemon can you add my pokemon in your next game you can change it to look more like a Pokemon

There are probably a host of legal issues to using Pokémon created by someone outside of their company. The legal rights to use it and the question of royalties comes to mind. Much safer and easier to use their own artists.

Not to mention that there are thousands, or even millions, of fans with their own designs they want added to the game. They could never add everyone's design even if the legal issues could be worked out.