While its a rough question, part of the answer lies in how people in America are thinking and how that thinking is changing over time. Of course, this affects the children in this country.
There are a lot of changing values compared to when this country was founded. People have gotten soft, and don't want (often) to share with their children the realities of life, and that life is often really about hard work. Children are often watching their parents or caregivers, and pick up bad habits.
It is no one thing, but I do notice that the things you mention are on the rise, which also happens to coincide with people thinking less about things, and not willing to think or work hard on many things. Its a different world with a different mentality, and this is one of the effects I think. A cause and effect, if you will. Surely, some are turning things around, and trying to break bad habits which is hard to do. It must be done if things are to change. Its like we aren't preparing kids for a real world where they will have to fend for themselves. You could add to the list, "why are so many living at home long term after they grow up...?" as well. Its hard on everyone, and parents need to relearn tough love. They are just resentful and the relationships are stressed anyway when we don't teach them how its going to be when they get out there.