I would guess part of the reason for this was box art used to matter because most of these cards were sold through dedicated electronics retailers like Fry's Electronics, Microcenter, and CompUSA. There was basically no such thing as online ordering for this sort of thing. People were physically browsing goods on shelves.
Just chiming in here, but at least two of the generations of cards there are from ~2005-2008 and we old farts definitely bought (or convinced our parents to buy) things from Newegg at the time!
I think it comes from a marketing exaggeration of what the card could do. None of the cards of the day could actually produce their own box art (in real time) but the art implies they could in a way they can get away with. It follows the tradition of box art on 8-bit games wildly exaggerating what the in-game graphics might look like and they'd sometimes post a tiny disclaimer in the corner.
I miss electronics retailers. Any hardware project nowadays requires me to wait several days before I can actually start as I am forced to order online.
