That would place the focus of the games on humans, which isn't likely to happen. Pokemon has always been about the Pokemon. Specifically explaining the origin of humans in that world really isn't important.
Pokemon evolution isn't the same as Darwin's evolution, it's more like a transformation. A Charizard though will still give birth to a Charmander. Basically, all Pokemon evolutions are like the tadpole-to-frog change, or caterpiller-to-butterfly.
I think it's just easier to say Arceus created humans as well, even if it doesn't outright state it in the games.