Their worry is that competition will be started by the people they've not hired and kept on the bench. I'd imagine a large amount of their top-level hiring is driven by taking minds off of the market, more than optimising their output.
Eg. Fuchsia OS might have been started because Google saw a bussines oppurtunity in writing an open source capabilities OS. Or, it might have been started to keep smart OS engineers from leaving the company.
Android has a bit of a better claim, as more of the userspace stack was written for it. It also wasn't started by Google (although they bought Android Inc. years before it was released publicly).