Follow the money. Foster care means employment for social workers, money for the operators of group homes.
It's also a big cash cow for some foster parents. What you see is girls "giving up" their babies to foster care, and grandma graciously taking them in as foster kids (they prefer to place foster kids with relatives). Reimbursement rates for foster kids is much higher than welfare payments.
There have been news stories over the years regarding gibsmedats “fostering” many more children than anyone could be expected to care for; it IS just a cash cow (and the stories are in the news because of criminal activity in the “family”).