I'm sure the competitive drive to dominate (in generosity) plays a role. Look at the rivalry between Rockefeller and Carnegie for example. Once they grew old and their companies either bought out (Carnegie) or broken up by the feds (Rockefeller) they basically switched to seeing who could get their names put on the most public buildings by building libraries, universities, etc.

What else is he going to do? At that level of wealth, the marginal returns on greed have diminished to almost zero.

So one must find another way to feed ones ego. What is left apart from fulfilling the god complex and saving the world?

Edit: has->have