Because it’s far easier for someone with a great vision about how to use the state’s wealth for everybody’s benefit to get themselves elected to the governing body than it is for them to get themselves reincarnated as an aristocrat.
I base this on the number of new billionaires vs the number of major new political initiatives.
I think it’s quite easy to have a vision, but very difficult to filter those through the will of the people. In the sense that a great vision is very subjective and may not be believed by the large number of people necessary to change things in a constitutional republic.
History shows that such people are much, much rarer than people who have a great vision about how to use wealth they have either inherited or built up themselves for everybody's benefit. Or, to put it another way, people with a great vision that will actually work are far more likely to become entrepreneurs than politicians. Bill Gates is eradicating malaria using his own wealth while governments have failed to do so for centuries.