Near-mandatory moving out is also a common attitude in Scandinavia, not a purely American attitude. If anything it's stronger in Scandinavia: it's pretty unusual not to move out by 18, and even seen as a bit weird not to, unless you have a good explanation for it (e.g. really sick parents). But that's partly because the high minimum wages and strong social system means that it's unusual to need to live with parents for economic reasons. So it's become a cultural expectation here that you won't.