Because they clog up slots that could be used by someone ambitious. If you've got 500 Major slots, but 400 of them are occupied by people who don't want to get promoted, then you only have 100 Majors who can get promoted. So they either have to spend less time in the job than you really want, because you need a certain number of Majors to get promoted to Lt Col each year... or you don't get enough Lt Col promotions, so you don't promote to Colonel as fast as you should. Repeat for every other rank.
That said, of late in the USAF, the promotion rate for every rank below Lt Col has been 95%+. And I think Lt Col has been fairly high as well. The issue isn't kicking them out once they reach a rank, but rather them deciding to get out before then.
That said, of late in the USAF, the promotion rate for every rank below Lt Col has been 95%+. And I think Lt Col has been fairly high as well. The issue isn't kicking them out once they reach a rank, but rather them deciding to get out before then.