why I buy antibiotic free animal products

The first and most obvious answer is concern about antibiotics resistance. The amount of continuous, prophylactic antibiotics we use on farm animals is suicidal. But the more complicated answer is that it's a simple way to improve a number of hard-to-measure outcomes. Any number of animal care sins- low quality food, bad hygiene, too many animals, monoculture- lead to disease. But fixing all of these is expensive, and antibiotics are cheap*. But fixing them improves other outcomes as well- a cow that has all the trace nutrients it needs to fight infection will pass those on to me when I eat them. It also forces you to use smaller and more genetically diverse herds, which I think has a number of benefits. So even though the optimal world might have occasional farm antibiotic use, the best outcome I can enforce is no antibiotics at all.

And you know, there's no reason we couldn't have a tax on farm antibiotics. We don't have to worry that it's regressive or that it will lead to more infections, because those new infections would be in the same farmer's herd and they have every incentive to do the optimum thing.

*To the user. If we properly taxed them to internalize the negative externalities, they would be much more expensive.