Because ten years ago GIMP was a good alternative to whatever version of Photoshop was out at the time. So those people ditched PS and used GIMP. But now Adobe has pumped tons of features into PS that the GIMP crowd doesn’t even know about, so they still think the two are still comparable.
I still use GIMP exclusively but I’d be lying if I said watching others use Photoshop didn’t make me jealous.