1. Why do we have bacteria break down our food instead of breaking it down ourselves? Especially, why do cows need bacteria to derive energy from grass, instead of some endogenous mechanism to break down and digest cellulose.
2. Why can't animals that ferment food with bacteria just keep all that shit in their guts, fermenting, instead of shitting it out? It seems like a waste to have to keep eating more grass, when the bacteria are what provide the cows energy, and when all the bacteria need are cow turds.

Basically I'm asking why our cells can't or haven't bothered to develop a way to do what bacteria do. Why don't we have specialized cells dedicated to breaking down food in our digestive tract?

Pic probably unrelated.