Half of the ~2 million owners have played the game for less than 33 hours. I'd say it's entirely possible that 35% simply haven't finished any games.
That's an interesting metric; I actually fit into that category for many other games I got off steam. For example I bought the $200+ Endless Legend bundle when it was on sale for like $16 or something. I tried a couple games. That game is actually very deep, very few imbalances, many ways to play which I didn't really get to see yet, but I could tell it would be an awesome game through and through. But still... I just can't seem to get hooked. I tell myself I can always save it for later... meanwhile I'm loading up some other game I've already explored thoroughly instead.
Steam is a platform that makes it too easy to just pile on a huge library, and never really get utility out of most of the stuff you buy. I'm guilty of this in many aspects, not just for games... ebooks, movies, gadgets, suits, firearms, you name it. If you select a random "thing" from my pile of irresponsible buys and made an achievement list for it, I'd be that guy that never even utilized it to 10% capacity. Chances are, I'd be the guy that never completed "Irish Heartbeat" for that random "thing"
Civ VI wouldn't be one of those things for me, but I can see how it could easily end up being one for some other irresponsible buyer. So to me, it's not really a metric about the game itself. It's more about irresponsible buyers. I think only a small proportion of steam users actually go out of their way to disable achievements.