The language and culture matter. The same salary can't buy the same amount of things in every country. If you were offered a job in France or Germany (or pick any European country that you don't currently know the language of) that paid 10 percent more than you make now plus the moving expenses, would you move for that job offer? Would you move if the job in Europe paid 50 percent more? There are differences between how people would enjoy life in Country X vs. life in Country Y that cannot be quantified into an amount of money. I would change your first sentence to "all other things being equal, workers want to make as much money as possible."
Sorry, MLS probably isn't losing money on its own at this point. It may look like it is because of the way MLS owners structure their investments. The Columbus Crew is reportedly paying an exorbitant amount of rent to the corporation that owns the stadium, which is owned by the same people. Other MLS team/stadium arrangements are probably structured similarly. Also, even if MLS were to get 100% of the money from the packaged TV rights, something like 75% of the league's revenue would still come from game day operations: tickets, merchandise, concessions, parking. And there's no evidence that MLS gets anywhere near 100% of that money. Finally, even if MLS were to owe the USSF something, I'm not sure how exactly playing a month-long preseason tournament with Chinese clubs would help either the players' fitness or the league's competitiveness against Mexico. Every team in the league played at least 4 preseason friendlies, and some played as many as 8, during the month of February 2016. The majority of MLS teams played at least one game against foreign opposition, and many of those games were against Latin American clubs. Playing a tournament with Chinese clubs would be instead of these games, not in addition to them, so would in fact rob MLS teams of the opportunity to face Latin American opposition in preseason.
Okay, yeah. But those are still economic decisions. You can be a rational economic actor and realize that getting a $3k a year raise, but you need to spend $10k to move isn't the best idea. Choosing which company to work for based on a meaningless off-season tournament that no one, in any country cares about is not that.