You could find a lot of capable people willing to do the job for a lot less.
But if you pay them a fixed salary, even $1m, that isn't directly linked to the performance you're looking for, then the game is to avoid anything too risky, even if the expected value is very high, if it might endanger the CEO's job. The game would be to take the minimum risk and do well enough to not get fired, and that's it.
If Tim Cook manages to double the stock price, he'll have $700m. If it drops to $100, he'll have $100m. It's a big difference, and his interests and risks are aligned with the shareholders.
Anyway, that's the theory. Give too much stock, and you're impoverishing the shareholders for little benefit. Don't give enough, and a superstar might decide he'd rather do a startup or an incubator LOL, or he may not want to pursue the same kind of risk-reward as the people who hire him.
$100m is still a great deal of money. I guess going to $700m, it's the difference between superrich for life, and a legacy of being a great leader and dynastic wealth LOL. Nice work if you can get it.