They make a lifetime supply of money each year, and plenty from non-league sources (endorsements, etc). Their energy is better spent learning how to manage the money they have, than fighting over who gets paid more of it.
I agree with you. But these are serious competitors and they have egos, so I'm surprised it doesn't come up more. You do see some of the guys at the low-medium end of the pay scale really happy to get to a level where they can provide security for their family, taking into account how short their careers are.