Hm, I don't understand what purchasing stock has to do with it. If someone bought the stock then they would own it. But that's not what happens when a company grants you stock - it's a promise to deliver you shares later.

If this hypothetical employee quit on April 10th, 2018, they would not get the 464 shares (== $668k) on April 16th, 2018. They also would probably not get a comparable offer if they quit to join Facegooglesoft. So, if they quit, they lose out on hundreds of thousands of dollars. That's enough to keep many people even if the work is unpleasant.