There's costs that aren't the PnL of buy-low-sell-high. The coders, the data, the infra. All cost money, a lot of money. Add to that there's a competitive aspect in the models interacting with each other, there's no slam-dunk.

There's at least a couple of MMs struggling these days.

True but I feel that the market maker label gets applied pretty loosely nowadays, couldve been a firm that mixes in a lot of intraday position taking trades that blew up