If economic conditions continued going well then they had the staff to meet their needs.
If they didn't then they'd cut the worst performing employees and anyone else they didn't want. Since every other company would also be doing layoffs they'd get no bad PR for it and low risk of employees leaving for greener pastures (since all other pastures would likely be closed).
Layoffs aren't some massive horrible thing for a company. They're just another part of doing business.
Because execs don't do all the hiring. Middle management and project teams go wild when no one is looking too closely at the costs and things can blow out pretty quickly.
Then growth slows and suddenly there's a need to tighten the belt a bit and a lot of waste is found
There was tons of uncertainty due to the pandemic so big tech over hired with the idea that if things weren't as frothy as expected they'd just get rid of the low performers/non-revenue orgs.
