Their strange stock vesting schedule makes me think that they're aware that people won't actually want to stay for four years. A back-loaded vesting schedule never benefits the employee, only the employer.

Other companies usually have an even schedule, for example Meta vests 25% per year (actually it vests quarterly instead of yearly). Google is an outlier too, but they do the opposite of what Amazon does - 33% in year one, then 33%, 22% and 12%. I suspect Google do this so they can list a higher total compensation (since initial total comp is salary, stock, and benefits for the first year), but getting more of your stock sooner is a good thing.