>And if competitors were in the same boat, why bother hiring these people at all?

Suppose an established org (N) got disrupted by a startup (N+1) because the startup was able to do things faster and cheaper. Once the startup becomes the established org themselves and slows down a bit, they are exposed to the same risk, unless they hire all the smart people (who would otherwise be hired by N+2 startup) and buy out all the N+2 companies who actually managed to do something despite having less money. Eventually it becomes too expensive and there is a good excuse to fire all those people. I don't think it's 100% of what is happening, but it could be.