All of the above, plus, at most (non-FAANG) tech companies, as an individual contributor your salary will plateau when you’re around 30-35. During your 20s you’ll get used to double digit pay raises and then all of a sudden that will stop. Even changing jobs won’t give you more than a 1-2% bump as some point.

I’ve had that surreal “we really want you to stay but we can’t give you a raise” conversation too many times in my career.

Yes, this. So then those young brilliant individuals move into management because it's the only way to continue some semblance of career progression in your 30s.

Then a subset of the gifted technical individuals grow tired of management, step down and go back to the tech to do what they really loved all along.

I've met a fair amount of brilliant engineers in their 40s/50s who have been through management and decided it's not for them and are now shining as engineers again after a decade lost to leadership roles

I would also add massive family debt/mortgage to this list. I saw so many star engineers fear-driven and stagnating in their tiny comfort zones to the burn-out or layoff.