The US have been good stewards so there has never been a reason to seriously contemplate moving en masse and without that critical mass you'd end up isolating your national network and having to bridge out anyway.

That said there is essentially zero technical reason why we couldn't just route (pun intended) straight around the US 'control' of the internet if we wanted so yeah it's a case of "we'll let you run it as long as you behave" rather than "you are the only ones who can".