Experience doesn't make cops good. A good cop with experience is better at being a good cop, but a bad cop with experience is better at being a bad cop.
Furthermore, I think good cops may be corrupted with time, but bad cops are never redeemed with time. So all else being equal, I expect experienced cops to be more corrupt than novice cops, and more effective at being corrupt.
I hate how much this whole thread is just everyone dripping with sarcasm.
The same reason we elect mayors but not every single single person who works in city call. Or cabinet members under an elected president.
Effective governance and public and administration still require experience. Elections are great for changing leadership who set policy, agenda, and direction and term limits help there. But they can't replace people with experience. We typically do have some elected positions with oversight over police.
This is NOT to say that there isn't a massive need for more accountability with police forces. But term limits ain't it.
Furthermore, I think good cops may be corrupted with time, but bad cops are never redeemed with time. So all else being equal, I expect experienced cops to be more corrupt than novice cops, and more effective at being corrupt.