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.
Salaries are also deductible, so they get the talent in their pool, off the market, adding a small % of value, whilst also being able to offset taxes on record profits.
It's a pretty nice win-win for them.