I rarely finish any games, most of them get boring about 3/4 of the way through, even the short ones. Once I understand a game's mechanics through and through, it doesn't hold much interest for me any more unless the story is interesting and I want to see its resolution. On the other hand if the system is interesting, I'm more liable to start again with different chars/classes to see how they play. For me, it's mainly about the mystery of a new system, the mystery of a new virtual world and its lore, etc. Things are only interesting to me so long as there's still some mystery to penetrate about them. Once I'm in the sausage factory and see how it's made and put together, once the illusion of solidity is gone, that's it.
I remember in particular with The Secret World, for a while, when I was in the illusion of working for the Illuminati, it seemed like the best game ever, but once that petered out and missions became repetitive and recycled, the illusion was shattered and the game became all cardboardey and Potemkin.
One of the rare recent exceptions is PF:K, which I've always really wanted to finish, but kept restarting because building in that game is so fiendishly interesting I kept wanting to try all the things, and mostly got only 3/4 to about 5/6 through with my fave chars. One day ...
The OP is particularly amusing though - I wonder how many people stopped PoE at, or just shortly after, Twin Elms? That happened to me too - interest just fell off a cliff suddenly. I'd bet the same thing happened to lots of people at exactly the same point.
For me, it would be more relevant to note which games I've finished - games like the Shocks, the BGs, the Half Lifes, Deus Ex, DA:O, TW3, etc.