That implies that there are thousands of employees, including very hard to achieve senior engineer level who work at a Amazon only because they dont care about making an extra $100k/yr or are in love with the company or can't pass interviews at any of Microsoft or Google or Facebook or one of the many local startups. I believe it for juniors who all quit after 2 years if they can, but seniors?

Depends also on location. Toronto amazon employees have lower salaries because there isn't enough local competition for talent.

Seem to highly depend on date of joining. Is it unreasonable to assume that someone who joined around 2015, when the shares were in $300-400 range got an offer with a grant of 1,500-2,000 shares for mid-level?

An L6/SDE3 that started a few years ago might do it with stock appreciation