That has nothing to do with our football not being international. That has to do with us hanging our hats on something other than sports. The World Baseball Classic (America’s past time and an international sport) is going on right now and we aren’t shutting down for it.
This is a 300+ million person country that spans 6 time zones. We aren’t going to shut down and have a “spectacular celebration” for any sport ever. For one thing you can guarantee that 100 million of them don’t even like that sport no matter what the sport is.
Russia covers more time zones than the US but effectively shuts down for Russian World Cup games, India has twice the population of the US and will do the same for Test Cricket.
It's a matter of national culture. America has Football and Baseball ingrained in their Culture, other countries have other sports.
America does the same thing with Baseball as it does with football, As you say, it's an international sport, but holds a domestic competition to unilaterally declare their own "World Champions".
And you can't even argue that American Baseball teams are the best in the world, I note The World Baseball Classic is being decided between the DUTCH(!) and Puerto Rico....LOL!