I think at some point, which Amazon passed a long time ago, you become too big to be able to 'notice' everyone; there's plenty of stories where there's employees, usually white-collar office workers, that do nothing for years if not decades and still get paid.

This is just one (very depersonalized) approach to not keeping 'dead weight' around, or optimizing staff to always be on an upwards trend. It's like an evolutionary algorithm; trim off the worst performers and the remainder will evolve to higher levels. In theory.