There's a large cultural bias *against* having your child sleep with you. Additionally, a breast-fed infant is a lot more convenient to feed in the middle of the night if he's already right there -- not so with bottle-fed. Since many people in America still bottle-feed their children, they have no advantage in keeping the child in bed with them.

Ours slept with us for about 6 months, at which point he needed to sleep by himself.