They don't though!
There's no need for it.
Chickens and cows are totally different. A baby chick is worth about a dollar, and costs about $2 worth of food to bring to full size. If you need more chicken meat, grow a 2nd chicken. Besides meat chickens are too large for their skeleton anyway, they really can't be grown any bigger or faster.
Cows on the other hand are worth/cost 100 times this. You protect that cow with your life, cause it could be your livelyhood. If you lose a cow it's a big deal. If you can get that cow to grow larger/faster it's worth a lot more because it'll cost a lot less to get to market size.
They probably use hormones to get the cows to milk too. Your wife doesn't milk just because you ask her to, you have to have the right body chemistry to do it, the hormones from pregnancy?! Estrogen.
You can induce hormones created by the body, or you can just put them there. What's the difference?
These aren't natural creatures, stop applying natural "rules" to them. they can't survive without us, we have done this all to them.
You want natural meat? Go hit a deer with your car and cut it up. Cows, chickens, pigs, we created those, you can't apply "natural" anything to them, they aren't natural to begin with.