In a sense US is still being settled. Populations are shifting and adjusting. The United Kingdom has had two hundred years to adapt to the effects of industria

In a sense US is still being settled. Populations are shifting and adjusting. The United Kingdom has had two hundred years to adapt to the effects of industrialization. The United States got in on that much later, with the majority of it happening post-WWI, and at that point the modern layout of the country had only barely been realized. As you point out the frontier was still very much a thing in the years immediately prior to that. There's still a lot of shifting within the US that you simply don't see in other countries. People are highly mobile, and cities like Austin and Atlanta have changed dramatically in character because of large-scale migration. The only country actively changing more than the US is probably China.