Fairness and efficiency. If someone is making significantly more money than you, they are either:

a) creating more productive value than you or doing something more in demand by society [strong signal you should join them!]

or

b) manipulating their situation for better outcomes unfairly or fraudulently

In both cases it's in the interest of the greater good to have these things out in the open.

c) just better than you at what you're both doing.