It's the natural result of "fire the bottom 10% every year".

If that's the rule in your organization, and you have a core group of people that actually know the systems and get the work done, you better make sure you have 10% padding every year, lest you layoff someone important and their friends all quit in disgust. And since competence and institutional knowledge is built over time, that implies a revolving door of new folks coming in and most of it not making it.