It happens way too often, but family reunification is the ultimate with regard to kids in foster care ...and policies and laws fall in line with that. If judges and social workers did what they *wanted* to do, most kids would probably never return home.
My theory is that many of these kids are meal tickets for their loser parents. The parents don't care about the kids, but they'll jump through whatever hoops they need to to get the kids back so they can keep collecting benefits from the government. After parenting classes, rehab, cleaning up the home, etc. they're given another chance. Again, and again, and again, and again.