EDIT: I see that my take was wrong and too narrow. Thanks to everyone who replied.
I can say from my own experience that when you hate your job, you're usually not very productive, either. Maybe selecting for people who want out is more efficient than it seems.
The take that only those who can get a new job will take the offer implies that people working there are only there because of a calculation that involves money and nothing else. I don't think it's realistic and I hope others don't live in that world - it sounds pretty miserable.
In a non-cynical world, a great exit package would allow those that wanted to do something else to do so.
Those that wanted to keep at it - because they're engaged with their work - would have colleagues that also want to be there. The company would have a happier, more productive culture. Everyone wins.
It might be that there is an element of this calculation wherein low performers stay, but those people are definitely more desperate than most at FAANG.