I submit that the human brain isn't equipped to handle control of multi-hundreds of billions of dollars cap and the working lives of hundreds of thousands of individuals. Particularly if you're morally suspect to begin with.
This is just one of countless obvious examples.
FDR is a very interesting case study. He had the country in the palm of his hand and could have cemented his (or his party’s) power permanently, but instead he left the republic intact.
Especially once you start icing out people who push back.