The US has the second largest total length of metro system in the world. It also has the third largest population and is rich enough to actually have built a lot of metro, which a lot of the other countries with large populations are not(the next ones by population are Indonesia, Pakistan, Nigeria, Bangladesh and the Brazil).
Also one needs to keep in mind, that a lot of these are just matters of definition. The transition between regional rail and longer metro spacing is somewhat fluent(Paris RER, Japans metros, S-Bahn and so forth). Especially the Great Society metros sort of act like regional rail. For example BART is often considered a metro, but Berlins S-Bahn is not.