Because a foreign country having a base in the US wouldn't serve any purpose. The US military is sufficiently powerful that it doesn't need other countries to have bases there to provide security and deterrence in the same way US military bases provide these things to other countries (or the US itself).

I'm British, but I think it's funny how people see the US military very unrealistically, especially here in Britain. We don't care to admit how much we depend on the Yanks. There are dozens of countries whose security is directly or indirectly dependent on the US military. In fact, there are countries that exist today that wouldn't exist were it not for the US military. Kuwait, South Korea etc..

Even relatively powerful countries often need the US to pick up the slack and provide capabilities that they don't have. I remember reading that the entire European Union combined only has 1/10th of the military capabilities the US has. It's funny how we criticise Americans for their militarism when it's the US military's power that allows us to be less militaristic. The US has had morally ambiguous military actions, but the fact remains that without the US the western world would much less secure. The US protects all of our interests and acts as the guarantee of western policies. When Britain talked tough against Serbia in the Balkans conflicts, it was actually the US that did all the work. If the US didn't have bases in Europe, the genocide in Kosovo would either have been completed successfully or would still be happening now. Europe, despite its supposed strengths, couldn't have hoped to stop the genocide without American support. The US basically did 99% of the work while European countries only provided a token gesture of participation even though the genocide was occurring in our own backyard.