As whites, short of a foreign adoption, it is very difficult for them to adopt black children. Liberals typically consider it better for black kids to stay in foster homes than be adopted and raised by whites.
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.