Without a doubt that stat is because these companies have grown so fast. It's not that people leave after a year, it's that they have added tons of engineers in the last year (and have done so repeatedly for the last few years).
I worked at Google. Turnover was really low. 10 year average stay doesn't sound unreasonable for me if you select only the employees that have left.
Maybe I misunderstand the metric, but isn't turnout calculated as the average tenure of people who leave? If the companies just add
more engineers, this shouldn't affect the metric (unless you also count the tenure of people who are actually still working there, but that doesn't sound like a good way to estimate churn rate...)
I worked at Google. Turnover was really low. 10 year average stay doesn't sound unreasonable for me if you select only the employees that have left.