They get zero media attention because they're utterly insignificant, but many countries still have an American football competition on some level, whether university level or just an ordinary (amateur) league. Like I said, it makes much more sense in the case of the NFL than in MLB or NBA, because the NFL is the only serious American football league, and as far as I'm aware the only fully professional one (considering that Canadian football doesn't play by the same rules). But all the same, calling oneself "world champion" indicates that one has beaten all foreign rivals who might aspire to that same title, not merely the domestic ones. If you haven't played any games against foreign teams, even if those games would be laughable blowouts, it's hard to see how you can claim the title. And if the only serious league is the NFL anyway, why would you need or want that title of world champion, instead of just NFL champion? Is this some kind of jealousy of the (association) football world championships?

As for the NBA, the growing importance of European and other foreign players into the NBA isn't a coincidence - NBA teams are still more likely than not to win games against even the best foreign teams, but the gap is not so wide that calling the Lakers "world champs" is in any way justified - not without some tournament in place to let the Euroleague winner and other competition winners take on the NBA champions. In much the same way that while the US obviously have the best basketball players in the world, and based on sheer quality should be world champions, they aren't world champions at the moment and will have to prove themselves just like any other team next year in Turkey.