To steelman the argument that wealth is earned, this kind of stuff tend to follow a power law. So a 10% increase in effort or talent can result in a several fold increase in wealth - especially when the effects of compounding interest are considered.
This is most apparent in sports or the arts. Being just a little bit better at baseball can be the difference between a million dollar contract and being stuck in the minor leagues.
Of course the question of whether we should want success to follow a power law is a different matter. As is the role of luck. Going back to the sports example, being born at the right time of year can be a huge, permanent advantage[1].
strenuous effort by their employees or are you telling me that the average billionaire works 50000 hours a day?