Some people are just messed up in the head to varying degrees.

Some people have descended into drug abuse.

However I think a huge , if not the primary, factor is that they've had a bunch of kids but the daddie(s) left. This actually seems to be the status quo in many areas of Philly where I live. They don't even refer to the men as "fathers," instead they are "babies daddy". or some variant on that.

So lets say they're at home and the kids grandma is raising the three kids (also very common). The single mother then needs to make $24,800 to clear the poverty line for the five people in the household, and that isn't happening.

You could argue that she should give the kids up for adoption. But the problem is that if the kids stay institutionalized the state has to pay even more money(I think), and I believe few kids who are older than babies actually get adopted. This is further complicated by the system actively discouraging mixed race adoption of African Americans.