Google is pretty well-known for maintaining a very deep bench-- if an engineer isn't effective in some role, they'll just move them to some other team. Heck, a lot of hires (especially 3rd/last attempt) go directly to the bench. This strategy pays off well when a new competitor (like Facebook or Uber) comes along and snatches up people in the lead roles, or if the company feels the need to pivot to something really fast (e.g. Google+, which drew from many teams across the entire company).

Moreover, hiring a long-term employee today versus tomorrow might save money for a place like Google, assuming we expect competition and wages to continue to increase.

It strikes me that if Uber is laying off software engineers, their company outlook must be very, very dire. A layoff is perhaps better strategy than firing the bottom 5%, which would likely preclude the departed from re-joining at a better time. But probably the wrong message to send to a cohort like software engineers-- a layoff is a legal confirmation that the company made a big mistake.