I would say it's mostly because they want to be able to say they have the best video card on the market at the time. xD
I mean, if you do a lot of 3D editing, game design, etc. getting a brand new top of the line video card certainly wouldn't hurt, but you definitely don't need to break the bank getting the biggest and best video card you can.
I personally just upgraded from an aging 7900GS, as it just wasn't cutting it when it came to doing 3D level design for UT3 mods and such, and i needed something better. In my case, the vid card i was using was almost 4 years old anyway, so i had a pretty valid excuse for upgrading.
Honestly, if you don't really do a lot of 3D intensive work, or are not a huge PC gamer, you don't really need to constantly upgrade your video card, maybe once every 2 or 3 years if absolutely necessary.