Amazon not being on the hook for severance for a large percentage of high salary workers, plus not having them on payroll/benefits for the weeks/months it would take to transition them out via PIP (including the management overhead involved per staffer) usually adds up to just about enough in savings to go out and recruit fresh, RTO-willing, naive talent much sooner.
Amazon doesn't really pay severance for PIP, nor are they required to pay severance for layoff. They would be required to do a WARN notice, but how would that be worse than giving people 4 months of notice about the RTO policy?