That's because the Federal Government has no real jurisdiction over the drinking age, see the 21st Amendment. It's almost entirely in the hands of the States.
Try having sex with a child where it's legal and you'll see how quickly the Feds enforce US Law to overseas actions. Hell, on a far more mundane level, it's illegal to purchase a Cuban cigar in Canada, even if you smoke it there.
Drinking age is a state law. The federal law doesn't apply to citizens but to the states. And it's not enforceable really, you just (as a state) don't get any federal funding for your highways if you don't make the drinking age 21. Which, of course, no state would accept.