Two benefits: them not working productively at a competitor's place is one, and them losing the skills and drive to make real-world useful things after a few years is another.
It is a no-brainer decision from a game theoretic point of view: these companies already have a top-of-the-line, best in the world product. Working on more products requires lots of inspiration, time and luck, and even in the best case all you get is just another amazing product. (I.e., just more of the stuff you already got.)
Using your money and fame to neutralize talent that could be used against you requires no effort or thinking at all, only money. These companies don't know what to do with their profits anyways.