Basically, because they're not wrong on the facts, and they're far from the first to realize that.

There's a decent not-meme-based story around GameStop's turnaround, and a number of actual investment firms saw it weeks before they did. Investment firms are betting against each other, as per usual.

Also, now that GameStop has all this extra value, they can actually improve, so the not-meme-based story becomes even more compelling.