Why do foster children move around so much?

I understand that the goal of foster care is supposed to be temporary care until the parents can get it together, and that sometimes it takes years (or never). Why then can't the kids stay in ONE foster home until reunification can happen, or why don't they typically go back to their previous foster carer if it doesn't work out with the parents? And why do they have to move with little or no advance notice? It's truly heartbreaking!