This is actually a pretty big "problem" for me, as lately there's been tonnes of games that look good and that I've started with, actually enjoyed and then just sorta...let go. Its gotten rather common lately as I've gotten older myself. Maybe its my priorities shifting or because I just cant bother, not having the same patience or determination.
Games that tend to stick out are big RPG titles. In fact, every single bethesda game. The only bethesda game I've managed to finish in the past 5 years is Fallout: New Vegas. Games like Skyrim and Fallout 4? Nope. I get about halfway and then I just loose myself in the open world to the point where I get bored, do something else. I eventually revitalize my interest in the game but by then it feels like it would be more fun trying a new playthrough anyway, a fresh start. And then it all repeats itself. I think I have 500 hours on Skyrim, but I havent finished the game once but have spread it out over 5 different playthroughs.
Other games include Dragon Age: Inquisiton, Wasteland 2 and Divinity: Original sin. And those are all pretty good games. I just cant be bothered to stick with them through the entire ride. Shadows of Mordor as well but I guess im one of the few people in existance that actually didnt find that game all too enjoyable, so there at least I had a proper excuse to shelfing it before I finished it.
The one really big RPG title that broke the pattern here was Witcher 3, which had me glued to the computer for countless hours and I didnt even remotely concieve shelfing it until I've completed it. There's...something about that brilliant game that the other ones just didnt have.