if the stock price is higher when they borrow and sell, and lower when they buy and return, they make a profit equal to the difference. they lose money if the stock price goes up. all these people buying GME and driving the price up will hold GME stock, and the shirt sellers will be forced to purchase back the stock at premium prices. even if the stock price goes down slightly, the people buying up all the GME will still profit.

but more importantly a lot of people's goal is to f over a major firm that constantly over leverages itself.