[tombert]

That doesn't surprise me; doesn't Citadel keep the entire bid-ask spread for every transaction they facilitate? Presumably between that and arbitrage opportunities that pop up from option contracts alone, I have no doubt that market makers clean up pretty well.

They wouldn't hire me either!

[eru]

Citadel has plenty of competition, eg from Jane Street. The markets for market making are some of the most efficient markets on the planet.