Carrying them to term arguably isn't the problem. It's the 18+ years of obligation and sacrifice that come afterward.
Adoption isn't a viable option. Black and Hispanic mothers who put their kids up for adoption aren't really putting their kids up for adoption - they're handing them over to foster care for 18 years. Adoption is the plan of last resort for most parents these days, after every kind of medical intervention has failed.
On what basis are you saying that they aren't really putting them up for adoption? If the parent wants the child to be adopted, then there are adoption agencies and other resources to help them through that process. The foster care system isn't meant for that - they place children who are already in foster care.