The relative usage isn't even the primary issue. The surrounding conditions are. Farms are ideal breeding grounds for quite a few reasons. Add to that rather barbaric practices that can result in treatment periods dragging on quite a bit and the result isn't at all surprising.

Antibiotics are not cheap. Make it cheaper and you will see how they will use it on livestock.

There are already regulations around this. There are new ones coming. It's very easy to test for compliance.

There is also a not insignificant portion of "animal antibiotics" that people buy to use themselves because they can't afford the 6x markup on "human" antibiotics plus the doctors fees only to get the same pill made in the same factory with a slightly different label slapped on it.