Internal mobility is strongly encouraged. People typically move after promotion, which is expected after 2 years. Some people don't like to move, bringing it down to about 2/year. All working as intended. Then, people moving out tend to want to join new teams, for the career growth opportunity, while you want new hires in established teams.

Note: that's anecdotal from a couple years of observation. I haven't looked into any Google-wide statistics (and if I did I would not blabber about them). But I'd be surprised if this was way off.