I doubt Google cares if the majority of it's people stay or not. Beyond the rock star employees who they absolutely know they need/depend on, they have an unending pipe of software engineers who want to put Google on their resume, in part due to the value of having the resume item and in part due to all of the perks.

Assuming the free food and massages will keep a lot of talent there, why would they bother incentivizing the average individual employee with interesting work and promotions? If you leave, they'll replace you, likely without shedding a tear.