They're rusty because they haven't played a high-intensity game yet this year. They're still relatively in preseason mode, and most of the players haven't played a full 90 minutes with the entire starting roster. If they have played a full match together, it's against a lower level team in the preseason. Even if they have a preseason match against a fellow MLS team, the other team is probably figuring out tactics, who works out best where, and fitness.
Yes, they do have time to rest, but their bodies aren't exactly at 100% fitness and ability vs. how they would feel if they were in the middle of the season, or even the first 5-8 weeks of the season. Games every 3-4 days isn't the easiest thing to do, but the Mexican teams are in much better shape for handling it at this point in the season, being that they're already 7 weeks into their regular season.