I genuinely do not understand why they don't do this. Bezos either has some sort of master plan relating to a high-turnover staff model or is delusional about the true culture underneath him. I don't know which.
In general, though, Amazon is always kind of starved for engineering resources. It sucks so much to work at in so many of its teams that it has a colossal retention problem and, one would expect, the corresponding level of technical debt.