The simple answer is that they hire more than they fire. In a lot of cases they will fill a role immediately after they fire the last person who was in it. Average employee retention at companies like Amazon (both voluntary and forced) is ridiculously low - something like 1.5 years. PIPs, forced burnout, mass layoffs etc. are all part of the corporate strategy. The revolving door helps keep costs low because employees leave before the bulk of their stock grants have vested.