I think you hit the nail on the head.... they are so comfortable in my house, that they haven't the motivation to pursue a higher paying (or any) job that would enable them to move out. I love my kids enough to not want them literally living in the street. ...
I've discussed this with my wife, the kids say they can't afford to move out but what it really means is that they couldn't afford to pay for rent, food, utilities AND still afford a car payment, cell phone, internet, cable TV and going out with their friends. I remind her that when we were first married we had a clunker car paid for with cash (no car payment), no phone, no computer, no TV and stayed home because we couldn't afford to go out. We still paid our rent, utilities and bought food. We didn't have a lot but we got by and were very happy.