It's not just about experience as an individual though, Google has hordes of arcane institutional knowledge about it's own platforms to maintain, like any large organization. Even if they were replacing everyone with senior devs and not juniors, you'd still be losing little bits of that each time someone left.

You see it at institutions, hundreds of individual "low bus factor" risks spread throughout the organization. The company would survive, but it would be a tough time, and a terrible place to work, during a large knowledge exodus.