It is absolutely not convenience. To flip it another way, these are often homes you would never allow to adopt a baby, or homes where the child is inevitably taken by child services due to abuse or incapacity of the parent(s). Then they end up in foster care, far too late to be adopted by the nice well-to-do family with a white picket fence, and they bounce around the foster system.

The foster system can't even handle all the kids that aren't adopted, and some kids end up too broken to stay in care. What happens then? At some point they just throw a bunch of broken kids into a shitty house together and have a child services worker come check in on them every day or so.