I can offer some rather cynical reasons:
1. If you've been teaching a long time, you get the honors/advanced classes, and your younger colleagues get the thugs.
2. In my state if you've got more than 27 years in, you can't be made to do additional duty such as hall monitoring.
3. If there is a shortage of classrooms, the ones with the experience won't be the ones roving.
4. If you've been around 20 years or more, an administrator who's been around less than that will not mess with you.
5. In my state, you don't collect full retirement benefits until 30 years of service have been reached-leaving at twenty years is only worth half.
5A: You can also retire, come back, teach half time, and still collect your retirement-again, welcome to NC.
6. If something happens that could get you fired, you would probably be able to retire before any disciplinary action even got underway.
See, there are some perks to teaching- you just have to be real real patient. My only worry- one day two decades down the road, the conventional wisdom will be something like:
"We've got to put our strongest teachers with our weakest students"
"Our newest teachers need a classroom to provide stability while they learn classroom management."
Then it will be back to thugs and roving. :?