Monetary rewards are far from the only rewards. Ability to effect change is a broader version of that reward: Elon Musk doesn't have as much money as Zuckerburg, but there's an argument to made that he's wealthier, since he's able to enact more/different change in the world.
(Of course, Facebook has had such a ridiculously large effect on the world that argument is probably hard to win, but it illustrates what I'm trying to say)