Because the US Navy is the one that has taken the British Empire's role in securing global trade, finance, and various technological services. It isn't simply that the US has nukes, the US is actually a useful country to other nations. It's the same way with China. Why can China take Hong Kong or Taiwan and not get sanctioned? Uhhhhh I dunno, maybe because other countries like the fact it provides them with a useful service. It's almost as if having friends... will result in you... being supported... by those friends... Whoa, that's a big concept right there.