If you fired all the people that build things, then your company could no longer build things. If you fire all the people that sell things, then your company could no longer sell things. Grug brain understand this.
But if you fired all the managers then... What? Wouldn't everything just continue as before? Grug brain has never understood management.
Then no high-level decisions would get made. Any technical argument that spans beyond one person would be at risk of running into an eternal stalemate. No strategic choices would be undertaken.
(Tone indicator: Not a value judgment. I'm not saying that all managers are particularly good at this, same as how observing that "builders build things" does not mean that the builders are necessarily good at building.)