I've heard the rebuttal that it's not necessarily that, but that getting jobs if they quit the one they already have is hard. So they stay.

Check out this Ask HN thread for example about how bad the Job Market is -

https://hackertimes.com/item?id=47988268

I think the broader issue is that Meta has been evil for pretty much all of history so you can’t rationalize remaining employed there as simply a factor of the current job market (ofc this primarily applies to more tenured Meta employees)

Fair point.