When I was younger, and therefore had less access to multiple games (say Christmas and birthday presents exclusively), I always finished games that I started. Beyond simply getting to the end, I also typically completed most or all side quests/optional content. I imagine this had a lot to do with not having anything else to play. Now that I'm a full-fledged adult and can (and do) purchase any game I'm interested in, I'm sitting on a few hundred Steam games, and a dozen or more PS4 and Xbox titles that I haven't gotten around to even starting.
Now not having enough time to start games isn't that unusual; I have significantly less free time now than I did when I was in school, as well as other responsibilities, but of the games I start I finish maybe 10% of them. The last game I remember getting to the end of (not even 100%) was Fallout New Vegas.
In the past 2-3 years I have started, really enjoyed, and then not finished: Borderlands 2, Deus Ex: Human Revolution, XCOM: Enemy Unknown, Stacking, Burnout Paradise, Halo 3/ODST/CE Anniversary, Lego Star Wars Complete Saga, Uncharted 4, Far Cry 4, Fable 3, and Arkham Knight. And those are just the ones I can remember off the top of my head. For each of them I'd say I got anywhere from 50-80% of the way through the entire story line before stopping. I certainly put enough hours into the above games to feel as though I got my money's worth, but not seeing the end of those stories sometimes nags at the back of my mind.