There are good reasons in response to: "Why we all think it's a great idea to leave our families at 18 and pay high rents so that we can live with strangers we found on the internet is beyond me," though personally I also don't believe that multigenerational housing is a symbol of failure, and often lets a person save a lot of money.
It's more difficult to maintain a romantic relationship when living with family (especially if both partners live with family), and it can also delay life skills (e.g. learning how to cook at home, clean, and generally learn to live more independently; though it's possible to deliberately learn this while living with family, it becomes a necessity to learn after moving out).
There are definitely some benefits to living on one's own for a while, but I do not think we should continue to view as the only viable option, or even the default for that matter.
Being pressured to move out of your parents' home ASAP is largely an American phenomenon I think. Maybe parts of Europe as well?
In many parts of the world, the point where you are expected to (and socially pressured to) move out of your parents home is marriage. Of course there are exceptions, like moving to a faraway city for a job, etc.
Whereas I see many American youths bleeding away their income on rent and expenses while living 10 minutes away from their parents' home.
It's more difficult to maintain a romantic relationship when living with family (especially if both partners live with family), and it can also delay life skills (e.g. learning how to cook at home, clean, and generally learn to live more independently; though it's possible to deliberately learn this while living with family, it becomes a necessity to learn after moving out).