Some quick googling indicates that this practice both increases meat production by 10-30%, and protects animals from a lot of suffering, as they spend their lives mostly healthy.
That said, yeah, we can't keep doing this, and we have to pay the cost. Let's just not pretend there is no cost.
(I'm not a farmer. I have no idea what the impact of this practice would be. However, as a naive person I definitely would strongly prefer to reserve the medicinal powers of antibiotics for humans or individually-sick animals, not huge animal populations.)
My only point is that this isn't done by stupid greedy farmers for no reason. It's a practical and profitable practice that has had a lot of short term benefits both for farmers, consumers and even farm animals. But it also has to end.