Antibiotics are not used for healthy livestock in many countries. The US is a very notable exception.
It comes down to cutting costs. In the US, it's not typical to clean the facilities between batches of animals like chicken. This mean that the new batch would contract any bacteria left behind the previous animals. It's not even that big of a cost, I'd call it borderline negligence.
I agree with you, this practice should end. Giving antibiotics to livestock at all is harmful and it should never be done as a precautionary measure.
On smaller farms, isolating or just killing the sick animals would be fine but that will not work in industrial scale.
So yes, it's about costs, but not for the reasons you cite.
But that's irrelevant, it's a cost cutting measure with harmful consequences to the society as a whole. The regulations and producer prices should be adjusted to make small to medium scale farming profitable and stop the antibiotics madness.