It's a good question - I grew up playing Dungeons & Dragons, but it could have more accurately been called Dungeons & Orcs or Dungeons & Treasures or Dungeons & Assorted Non-Dragon Monsters.
I don't think I'm alone in this experience - the Dungeons & Not Dragons experience.
Lots of us are probably guilty of saving dragons for later.
There are good reasons (dragons are scary and dangerous, so starting adventures can't handle them, it helps to have some buildup) and some not so good reasons (we hold them back for the "right" situation, or the "right" level of power of the adventurers). But all the same, it's really common to have dungeons and not nearly as common to have dragons.
Over on Google+, I wish I could relocate the thread, someone talked about running a (Pathfinder?) adventure for a 1st level group that featured a dragon-slaying weapon and a dragon you could kill. I remember the reaction of the veteran gamers being, basically, "lame;" that of the newbies, "Cool!" But I think, geez, wouldn't it have been cool to fight dragons early on, even if it was risky and/or required special assistance and/or turned out to be a fairly weak dragon? Maybe not at 1st level, but is it bad to get into fighting dragons very early?
I used more arch-devils and demon princes in my games than dragons. That's kind of sad, really - it's never a bad time to bust out a demon lord but I have to save dragons for the optimal moment to use one?
My current DF game has a dungeon with dragons in it, but the PCs haven't reach the level(s) where dragons are. So even I'm guilty of it, even if I'm more complicit than solely at fault.
Still, I'm starting to think I'm still doing the thing I'm bemoaning here - saving the good monsters for the end. Back-loading the fun.
Something to think about - myself and others. Why don't we put more dragons into the dungeons in our fantasy games? Why does putting dragons front-and-center, early and often, feel like a bad thing? Should it?
I'm starting to think that, no, I should have put them more into the spotlight into my DF game from day one.
I won't move the dragon or dragons in my dungeon, because that would feel lame, but I kind of wish I'd placed them for easier, earlier encounter. Maybe I should have put a dragon on level 1, honestly, and let the PCs work up to tougher dragons . . .