Cost of living. In the U.S., most college graduates can't afford to live on their own in major cities. Jobs require ever-more qualifications and advanced degrees. Productivity goes out the door and escapism comes in when you're stuck with your parents.

That housing bubble was horrible for the economy. Far from a sign of good things happening, it meant owning their own home was out of reach for the vast majority of young people. The amount of productivity and joie de vivre lost cannot be overestimated. Being stuck living with people you don't want to live with is toxic (that includes non-family members, too).