Plus, wouldn't you sometimes prefer experienced people in office who know how things work rather than every few years getting newbies who have no clue as to what's going on, only to have them forced out of that position by the time they finally have a handle on how everything works?
Besides, we do have "term limits": it's called ELECTIONS.
Just because some people are too lazy to actually go and vote bad politicians out of office when they're up for re-election is no need to put in place laws / rules limiting how long a good, effective politician can stay in office.