You're holding your gun to his head: this villain, this monster you've been tracking for days, weeks, maybe months. You've used your wits, your talents, and every last resource getting there, and your finger is on the trigger. Why the hell aren't you pulling it? Why do you still have that save game, right before the end boss, unfinished?
This is something a lot of gamers run into, I think. Pondering it, I've come up with some possible reasons.
Grandia II: There's still a save-game somewhere, right at the end boss, waiting...
The first reason is one that crosses over from other media: It's so good, you don't want it to end. Some games are just fun to play and play. Morrowind was like this. I spent 80 hours with my stupid tiger (he wore a hat) and never finished the game. I enjoyed the gameplay itself so much that I just kept toolin' around, and by the time I eventually got tired of it, I still hadn't beaten it. I made up for this in Oblivion and Fallout 3, so Todd Howard doesn't have to hunt me down in my sleep.
I think another explanation for this--coming from my experience with Final Fantasy X--is the "effort" factor. After about 48 hours of the game, I asked myself "Do I really give a shit about any of these people? Other than Yuna being cute..." The answer was no, and I quit playing. I don't really regret it, either.
The third (and easy) one, is of course, distraction. How many games on the shelf lay unfinished because another, more interesting game came along? The only difference here is the distraction's timing.
I'm sort of just babbling here; this doesn't really have a point. Can you think of any games you've left unfinished, mere minutes from the end?
Some games I never finished:
Final Fantasy IX: didn't give a squat about finishing (both times)
Fear Effect 2: can't get past the wall-monster chase
Xenosaga Ep 1: stuck in last dungeon
Star Ocean 3: can't remember where I left off and game is no help
Dragon Quest VIII: got distracted by other games
Infinite Undiscovery: can't get past certain level in final dungeon
Anther reason I think games go unfinished is that the game itself is not conducive to completion; it screws you over at the last minute and keeps you from winning. You've been perfectly successful with little resistance up to the end and then *WHAM!* you are unable to gain levels, you run out of money/items/both, you aren't strong enough to go on, and you aren't allowed to leave to level up/buy items/gain money/etc. (whether the game plays a fun games with save points or it decides you can't leave the last area once you enter). Also, action games can have parts which are nearly impossible to pass. I mentioned Fear Effect 2- I also got constant game overs in FE2 from not being able to outrun an explosion earlier in the game. Being quite capable of moving a joystick and holding "run" at the same time, I really don't know why I kept failing.
I have trouble finishing the Final Fantasy games. I've played nearly all of the rpgs and only beaten IV on the DS and X on the PS2. I think you nailed it about the character development. I didn't really care about the story at all, and was only playing for the fun combat system and levelling mechanics.
I'd beat the penultimate boss and lose interest. I remember this happening with VII, VIII and IX. Another factor could be the ridiculously intricate final boss battles. How many forms do I have to beat-down before the game-over screen? I think one or two is enough.