How do tech giants manage turnover costs? Lately I was struck by different studies concluding median job tenure at Google, Facebook, Amazon, Microsoft and Apple is 1 to 2 years, very low compared to the Bureau of labor statistics which is a little over 4 years average job tenure.

Sources: 
DeepTalent
Payscale
Paysa

Turnover is uniformly described in literature as costly and avoidable. Given the fact they have near unlimited power for generous policies, that they let turnover get this high comes as surprising, possibly is a result of a cost calculation.

Is tech giants turnover cheap or unavoidable ? How do they manage turnover costs, generally speaking ?
Companies like Google and FB are talent focused. Turnover is high for a few reasons:
 1). Because the employees tend to be VERY good at what they do, thus their opportunities are vast; and people tend to gravitate towards who pays the most.
 2). Stress. 
 3). Contracts. Many employees in these big firms tend to only have short term contracts for specific projects. 
 4). These companies demand some of the best talent and work, and if an employee isn't reaching par consistently, no reason to keep them.
I'm sure there are more reasons, but those are researchable. =-) 

Hope this helps.