Soy isn't very appetizing to humans in large quantities, so if it wasn't for animal feed, we wouldn't' grow nearly as much as it. Likewise for corn, which is a not so great staple for humans, and often just used as a sugar substitute. Yes, we can live on animal feed, but we can't really thrive on it, and a lot of land is only suited to such.

Meat is still a useful medium for converting what humans can't consume very well to something they can. Especially pigs, which in much of the world are used as recyclers (not in the west anymore, due to safety issues, and probably not for long in the undeveloped world, but they have bigger problems to worry about).