GIF has a certain mindshare that the other formats don't, crossing over into folks who are less deeply technical.
I've worked with a couple web designers and developers that said "Gif" to mean any image, as in "Move that gif up twenty pixels" when referring to a logo that's actually JPEG. I think it's a holdover from the days of BBSes and AOL and CompuServe, where everything really was a GIF. "Check out this new cheerleader Gif I got." It's not a far leap to imagine these users just out of habit and familiarity clicking GIF in the save-as dialog without really thinking about it. So GIF persists out of familiarity and inertia.
I've worked with a couple web designers and developers that said "Gif" to mean any image, as in "Move that gif up twenty pixels" when referring to a logo that's actually JPEG. I think it's a holdover from the days of BBSes and AOL and CompuServe, where everything really was a GIF. "Check out this new cheerleader Gif I got." It's not a far leap to imagine these users just out of habit and familiarity clicking GIF in the save-as dialog without really thinking about it. So GIF persists out of familiarity and inertia.