I'm far from an expert, but aside from costs of living, the driving factor might just be younger people have been indoctrinated into a "school/college/work" pipeline. This path might discourage/postpone career exploration and hinder kids from figuring out what want to do. As such, young people stay with their parents as a sort of buffer while they formulate a more concrete life/occupational goal. Nothing wrong with younger generation; just the best coping strategy for a system that tries too hard to be one size fits all.