It's nothing to do with correlating the wages, but more the structure/hierarchy of football. Internationals are not as regular and are a risk to national league fans, that's not just you Americans either, you have players in all the major leagues in the world including over here in the Premier league which is one of the most physically demanding in the world, it also generates the most revenue out of any football league in the world. By calling them up they are risking serious money and the clubs position in their respective league and you're say that is irrelevant?
I'd also like to reiterate that it's not just MLS
It's the premier league, la liga, Bundesliga. America have top class players in the best leagues in the world, MLS is growing but some of these players are risking a hell of a lot more