Why would it though? In every other industry, union workers make much more than non-union workers. And unions don't seem to drag the salaries of other high earners (actors, screenwriters) down.
This absolutely happens in MLB/MLBPA. Players are not allowed to negotiate most types of incentive-based contracts that would align interests of management and players to give them higher possible pay, amongst other methods of capping pay to improve standards for average professional athletes.