Lots of reasons come to mind: Ease of access, marketing, tradition, convenience. Western culture is very disconnected from the sources of our food. The science on the benefits and risks of eating meat seems to conflict, from what I've read, but it's clear that the environmental impact of cattle-produced methane is a real problem. Biodigesters seem to be a way to help address that. I remember reading about how rain forests are being cleared to make room for more cattle, as if we don't have enough. That just seems crazy to me.