At least one reason is that the stock has done so well. A typical software engineer hired 4 years ago is incredibly well compensated at Amazon because of this.
Consider a mid-level software engineer hired 4 years ago, on April 16 2014, with a salary of $100k. A typical stock grant at hiring for a mid-level software engineer in 2014 might be $400k over 4 years, backloaded so most of it comes at the end; on April 16 2014 the share price was $323, so the grant would have looked something this:
Apr 16 2015: 154 shares @ $323 = $50k = $150k total comp
Apr 16 2016: 248 shares @ $323 = $80k = $180k total comp
Apr 16 2017: 371 shares @ $323 = $120k = $220k total comp
Apr 16 2018: 464 shares @ $323 = $150k = $250k total comp
But the share price has gone through the roof since then, so things look more like this:
Apr 16 2015: 154 shares @ $383 = $59k = $159k total comp
Apr 16 2016: 248 shares @ $625 = $155k = $255k total comp
Apr 16 2017: 371 shares @ $901 = $334k = $434k total comp
Apr 16 2018: 464 shares @ $1,440 = $668k = $768k total comp
That mid-level, totally normal software engineer is making more than $750k this year because the stock price has gone up so much since 2014. They're likely to put up with a lot of bullshyt to get that kind of financial security - that's not just a down payment on a house, it can be the whole thing in cash in the east side suburbs, with great school districts.
Amazon (and Seattle in general) could be in big trouble if the stock price turns around. I don't think that's likely, but it's kind of scary to me.