What makes you think slaughter houses were needed? That just made things more "efficient". Humans and our ancestors have been eating meat since we have been a species.

Some of the things done in order to achieve efficiency has tangible consequences on the healthiness of the food. For instance cramming many animals in a very small space (e.g. in poultry farming) increases chances of contagious diseases developing. Industry is trying to compensate using antibiotics, but that has issues also.

It’s processing, which is what you were knocking meat alternatives for.