I don't understand the question. Are you asking why someone wouldn't abort their child?
There are institutes where they can be housed and cared for, instead of having to care for them 24/24, at least where I live. Most end up there by adulthood anyways, considering how hard it is to care for adults compared to children, especially when you're aging, and come back for week ends or the like.
Yes, there are programs in most states to help the parents in this sort of situation, but most of these programs let the benefits run out when the child reaches around 21-25 years of age.