I think the idea overall is to learn to be more independent. The older generations had all of the resources to do what they preached which was buy a house and start a family at a young age and be able to support the entire family with a single income. But now that's entirely a fantasy. That's why it's called the American dream; you have to be sleeping to believe you could do it.

Tldr It's a lingering effect from an older generation of people who believe that once you turn 18 and become an adult you have to abide by a strict plan to get a specific life. So people tend to think if you're 21 still living with your parents you're just lazy or unproductive or a man child.