While the actual cash received when you sell a stock to someone else does realize your profits, that doesn’t mean they lose when you win. The only way you get a zero sum situation is when you have matching longs and shorts. In the stock market most longs do not have shorts on the other side, so it’s not zero sum. Forex trading, however, is as MattW2009 outlined.
Market making is the same in all markets. The MM takes the other side of whatever trade, attempting to offset in an opposite transaction elsewhere.
The difference in the stock market, though, is that MMs must carry inventory since there’s an actual exchange of assets in equity trades - unlike futures/forex which are just agreements for future exchanges.
If every bank thinks they’re making money trading knowing that some other bank somewhere has to lose, why do they bother at all. They must know the losing side is going to come round to them eventually?
Because they know from testing their trading methods that they make x% over time. Also, they have far more capitol to trade with, so even a small percentage return still equals a good amount of gain.
Just like any good traders method should involve a good win/loss ratio + winners being greater than the losses.