If I own Google, I’m not willing to run the experiment of hiring a thumb-twiddler to run my trillion-dollar company to save a few hundred million dollars. I’m willing to pay an extra hundred million dollars to hire someone who appears to be 1% less likely to be a thumb-twiddler, even though I don’t have an oracle that can measure candidates perfectly.

In reality, there are few people who have had to opportunity to show their ability to run Google. You have the small set of people who ran tech companies as big as Google, or who ran big parts of Google. Those people are in high demand, which is why hundreds of millions of dollars is the market rate for the most appealing of the bunch.