The idea that if you're still living at home that your parents are still taking care of you. That they still make your food, do you laundry, pay your bills, etc. there is also a stereotype that you're emotionally stunted since you haven't moved out and had to take care of yourself. This is often summarized in the neck beard living in his parents basement meme.
Well, yeah, but there was a time when you could buy a brand new home for like $125.99, and at that price point living at your parents house would have been weird.