Agreed. But why support Trump and take the job?
Sessions is 71, iirc, and had been a Senator for a very long time. It’s not unlikely that he saw the office of AG as the crowning apex of his career. Being a Senator had been such a smooth and rewarding ride for him, it’s quite possible that he didn’t realize how challenging and potentially unpopular being Trump’s AG would be.
I read a report that Sessions didn’t know Rosenstein was going to sic Mueller on Trump until it was a done deal. It’s another possibility that things spiraled quickly out of control for the timid, passive Sessions. He hangs on because he’s come to resent Trump’s criticisms and he may hope he can still create some kind of legacy for himself.
Jmo.