What happens is that when a pure software company is a startup it is severely constrained in what it can execute on. As it gains traction, it receives funding

What happens is that when a pure software company is a startup it is severely constrained in what it can execute on. As it gains traction, it receives funding and revenue that allows it to invest in more people to get more done. In the beginning the marginal ROI of each hire is high, but as they grow it slowly decreases and may become dominated by communication overhead if proper structure isn't added. But as long as growth is on pace, they will keep hiring to stay ahead of the curve and squeeze as many gains as possible. When VC money is at play, management will be pressured to grow at all cost, and worry less about the downside because the investors are looking for the grand slam. There's never any shortage of internal stakeholders who will have ideas of where to put heads to good use. In short: they have 6000 because that's what leadership thought they could afford. Obviously a 10-year bull market leading up to the current lockdown situation puts them in a rough spot.