It's the same reason every year of your life seems shorter than the last. Because the amount of genuinely new experiences you have shrinks. When you were 5 years old, another year would be as big as 20% of your experiences thus far. However, at 29, that same year would only be as big as 5%. If we are exposed to new things at the same rate as before (We aren't, as explained later, gaming has slowed down), the overall differences are going to much, much smaller because you have a far larger set of experiences to draw on. So what might have seemed fresh and exciting to you as a kid, now seems boring a derivative as an adult.
It's not all just our age though. The industry went from change incredibly quick with the dual growth of the internet and computers, to only refining itself. I mean, as kids, we saw gaming go from 8 bit Super Mario, to games like Quake (10 Years) in the same amount of time that EQ went to current MMO's (If you regard "WOTLK" as "Modern" WOW--about 8ish years?). Does anyone here believe the EQ to current MMO jump was as revolutionary and different as going from fucking Mario to blowing people's heads off on a lan in Quake? I mean, the Mario to FPS jump introduced radical new elements in both play, graphics and systems--the EQ to modern MMO jump, or the Quake to BF3 jump just had further refinements to those elements (With a few notable exceptions, like environmental damage ect).
The changes just haven't been as big--The industry was reinventing itself every ten seconds back then. And your brain loves new patterns, it revels in them. The gaming industry gave you shovel fulls of them, because technology was shifting so quickly. The last ten years or so though, technology has refined itself, and those systems/patterns have become better--but they've also stabilized. Us being part of that first generation saw these rules and patterns for making games being laid down, all these experiences were tested on us--so every new game that uses them feels a little worn out the moment it gets booted up.
That's my feelings, anyway. MMO's and online games kind of stand against this--because other people create randomness in content, which can't really be accounted for. But it feels like, especially since the last few generations, the number 1 things designers have done is reduce the randomness another person can have on your play--which for new people, is great, because the game itself is providing that stimulation. For us older gamers? Not so much. But we're not the market. There is a huge, huge group of people that just started gaming over the last decade. And unfortunately for us, it's easier to refine the old stuff down, make it more accessible and sell it to those people, than it is to build new stuff.
I don't think we're doomed forever though, just for a long while. The next great leap forward for us will come with very advanced AI (For everything from mobs, to dynamic building), or new network/MMO tools (To allow for smaller publishers to build), or with revolutionary interfacing technologies (Kinect ect is the first stab at this).....If any of those three things really blows up, gaming will get it's magic back for us.
I know, personally, as cool as a VR tool would be, or niche MMO's, I really want a break through in AI. I can only imagine how much better games like Skyrim, or even Civ 5, would be if the computer was as complex as a player. Like imagine an open world RPG where each of the NPC's behaved like humans--it's a near impossibility to have this in an MMO because real humans need to log out, and have lives...But imagine if we could have it in a game, where NPC genuinely thought like humans (Like say, could understand language, so we could type and ask them open ended questions). It's insane to think about because AI has really lagged behind almost every other facet of gaming (For good reason, it must be extremely difficult to make.)
Sorry, rambling.
TL
R Obviously there is a lot of room for growth in the industry. I've kind of fallen out of love for games though because I see companies are specifically not growing, they are refining their product. But I've already grown bored with their product--no matter how slick and refined it is, I've already played it. Someday, when the overall market is as saturated as us, we'll get the next big thing.