There's a lot of wastefulness in animal agriculture that we don't see. For literal, physical waste, there is the problem of what to do with the huge amount of animal excrement. We generate far more than we could ever use as fertilizer, and much of what is leftover sits in "lagoons" and leeches into waterways. It's super eutrophying too, which means it kills off a lot of what's in rivers and leads to algae blooms.

Then in the non-physical category, it's a wasteful way to create calories because animals require so much input feed to create one calorie of meat or other product. Eating plants directly is more efficient.