A large part of the appeal of the franchise is the Pokemon are a part of their world, that they form much of its ecology and are living, breathing creatures with their own unique traits and personalities that humans can raise and form lasting emotional bonds with. It's a central theme for much of the franchise's narratives, both in the anime and within the games, and they try various ways of incorporating it into the game's mechanics and features as well.
It's rather Miyazaki-esque in its approach to Nature, of which the Pokemon are typically symbolic of.
Plus, I can't see people who are at all invested in the franchise wanting a live-action movie that comes down to simply watching people play a more elaborate version of the game.