It’s pretty simple, actually. Once such a dominant market position is achieved, you can get away with almost anything, whether with customers or employees. This is true of all the BigTech companies.
It doesn't need to be sustainable in the long run: just needs to get to the next quarter and there continues to be enough desperate people in the US or India willing to be ground up in the machine for a chance to buy a house in a major metro
(Source: I was at Amazon for 10 years, finally quit last month)
The only time Amazon was forced to change its ways was during Covid hiring boom where they couldn't compete in the talent market. They were forced to increases their salary bands and the culture was also a bit easy during that time. But starting mid 2022 it's been an employer's market and Amazon is making sure to juice every bit out of its employees while it can
Engineers have to spend an inordinate amount of time on "managing up", which means they have very little time and attention to do what would otherwise be a reasonable workload. Additionally, good engineers hate and despise this so it contributes a lot to the burnout.
It seems that at L6 and below workers are a Taylorism-style fungible widget driven to convert salary into work product, guided to create the most output for the longest time before mentally breaking down, then being swiftly replaced, with L7 and above being so incredibly political that keeping the snakes and vultures from eating your team is a full time job at every level of senior management.
It never made sense to me how such a ruthless and inhumane culture is sustainable in the long run.
I would love to hear positive counter perspectives from Amazonians because the anecdotes from my L6-L10 friends describe what sounds like an inhumane hell on earth.