I beg to differ. Go back and play Grim Fandango or Sam and Max and tell me they aren't still fun.
I recently went back to play Sam and Max and I grew bored within 5 minutes of starting the game. Adventure gaming simply doesn't cut it for me anymore. I need something more involving. Sure, adventure games are ideal for good storylines and characters, but if the core gameplay isn't interesting, it's all for naught. Ultimately, adventure games boil down to pixel hunting and trial-and-error, neither of which is particularly fun.