The culture wide spread in many parts of the US pushes people out of the house around the time they become adults to a point where it people might have looked down on you if you didn't (e.g. typical stereo type of the incompetent loser living in their parents basement).

While this clearly never applied to all of the US it applied to a significant part of it.

Now if culture would have massively changed wrt. how family constructs are seen that could explain the increase but AFIK there was no such change.

So in my opinion it's pretty clear that a main driving factor of this increase is that living in your own household has gotten dam expensive. Too expensive.

And yes you can re-frame it as "they don't want to" (because it's to expensive). But that misses the point.