Have 6 improvements at once" was achieved by 81,1%. Which probably means that close to 20 % never actually played the game at all. Going through my Steam library, I have 15+ games I've played less than one hour or not at all. Some of them are probably awesome games that I will most likely enjoy when/if I play them. So I would not necessarily conclude that most players who never finished a game had issues with bugs or were bored towards the end-game.
I think some people load up a game like Civ 5 or Civ 6 and just build a bunch of units to conquer stuff. They aren't really interested in building things, they just want to destroy stuff. They are either too young to fully grasp the game, or too immature.
As for games I haven't finished on Steam I'll go through some games with low play, I at least try to play all of my games I buy, but some games I haven't really gotten into like:
Sid Meier's Pirates: It's a little more action orientated than I thought, I suck at ship battles especially. 110 minutes played. Not sure you can actually finish this game
Xenonauts: 8 hours played, did not finish
Xcom2 15 hours, did not finish (though I may play again some day)
Sid Meier's IV: Colonization 9 hours, did not finish. I really could not get into this game despite loving Civ4
Sim City 4 Deluxe 19 hours, only paid $20 for it, so I think I got my money's worth
Microsoft Flight Simulator X: 2 hours. I suck at flying apparently
Torment: Tides of Nuenerra and Divinity: Original Sin 2 I still have not finished, but I may someday.
Every single one of my games has at least some play. My least is Sid Meier's Pirates. Only paid $5 for it, not so bad.
25% finished The Witcher 3
some games such as Witcher 3 are GOG games as well, and I have the GOG version. But I did finish the game once. It's just too long of a game to play again. I really hate overly long games. Witcher3 is the definition of a game that is too long. It's overrated imho.