I can't speak to everyone, but a lot of young people need to leave their parents' house because it's in a region without a lot of good employment. Some people grow up in rural or rustbelt areas.

For example: my parents don't live on a farm, but their house is between a corn field and a forest. The only work in the vicinity is farm labor, local gov, or retail/service. I guess a remote job could be an option, but their only internet option is slow unreliable DSL (cuts out every 20-30 minutes, phone company unable to diagnose the issue).

The nearest city is about a half hour away. It's relatively small, but does have some opportunities. No tech scene though, of course. There are a couple of small factories remaining, some banking/insurance business, hospitals and medical, and more retail/service.

My dad was an electrical engineer, and he had to commute 1+ hours each way to get to his office. Even if I was willing to commute that far, that wouldn't be an option since that company left long ago.

My parents do have an absolutely gorgeous view of a scenic rural valley outside their front windows though, and 100s of acres of forest behind them.