Nobody's getting heatstroke in Google's fulfilment centres, because they don't exist.
[1] http://www.collegehumor.com/post/7035742/office-job-vs-retai...
By comparison, Overstock.com averaged five years for developers between 2010-2013.
There is a separate tool that tells you the seniority of the people that are leaving. And while there is definitely a statistical bump right after the 1 year vesting cliff, even a cursory glance shows that the average and median final tenure is quite a bit longer than 1 year.
In my teams, representing less than 20 people in total, firings were far more common that people leaving. Two business people (marketing focused) left, two developers transferred to other areas (early AWS), while four developers and one business person were fired over the four years (fulfilling the "making hard choices" performance review requirement).