1- Drunk on Power?,
2- Blackmailed into maintaining control to do the bidding of their masters?,
3- Too stupid to retire and enjoy their accumulated wealth?,
4- They're in on the One World Gov't plot?
5- They really think they're smarter than everyone else and world just can't live without them?
6- All of the above?
If you read the debates on the creation of the Senate they are very enlightening.
James Madison was the proponent of a six year term as it would allow Senators to be protected from short-term pressures.
Quite a few other Founders opposed the idea, worrying that the Senate would become a national aristocracy.
Madison got most things right, but on this one history has proven his opponents correct.