Its just question of risk/benefit ratio, benefit is clear: cheap meat, because producers will be less impacted by deceases. Risks are not so clear in this case.
It doesn't stay localized; runoff from farmland is a major issue. In other words, the farm animal poops out a bunch of antiobiotics, then it rains and that poop ends up washing into the river/lake/water table. That's already something that happens with situations like E. coli contamination. Things that happen on the farm don't stay on the farm.