Video games have always been about innovation. That's why.
There are third rate platform games that are as good or better than some of the best platform games from the 8-bit/16-bit era but they don't get the attention because they feel "old". We don't want to see the same movies over and over again. We don't want to play the same games over and over again. The gaming industry thrives on creativity and "newness". If we don't have that, all we have are prettier versions of what we already have. Graphics get old--sometimes immediately after the initial shock wears off. What's left is what's important.
Every gen has offered compelling new ways to play. Be it complexity, Mode 7, motion controls, online, transitioning from 2D to 3D, or what have you. That's what has kept us engaged. Even a game like Grand Theft Auto III, Gears of War or Call of Duty can come out of nowhere and take a simple mechanic and tweak it, setting the pace for an entire generation. But after a while, that starts to feel like "been there, done that" and it's time to evolve, again.