Why is that?
Don't worry. The idea has more merit than most of what I've seen posted here.
Why? For multiple reasons. 1) It's never going to happen. 2) The only way for it to happen is to forbid incumbents from running which would be non-democratic. 3) There are a LOT of good incumbents. 4) It's a stupid solution. If I have a spot on a shirt, I don't throw the shirt out, I wash it.