Unfortunately US Soccer runs both the National Team and MLS to a large extent and as a result wants each to build on each other. This means they put a yes man in Berhalter in charge who won't do what really should be done and just cut off the MLS except for extreme cases because the league generally isn't even Championship standard and isn't going to keep players at an international level. Playing Concacaf sides should be a step down for the US players and it hasn't been that when MLS supplies the squad.
To be honest MLS has mostly helped all (excluding Mexico) the other Concacaf teams. MLS is a higher standard than their home leagues.