> Till then whats wrong with them staying with the family?

Leaving the family is good for many reasons. It teaches you to be independent, to live on your own, to handle your responsibilities, manage your earnings, i.e. to become an actual adult instead of relying on your family to cook, wash and do all/most of the chores for you. You can't get ready for a family and provide for them if you can't do these kind of things on your own in the first place. It's all about building experience and self-reliance.

Of course every family is different, but in society at large having kids staying longer with their parents definitely delay actual adulthood.