Possibly misleading statistic, as even achievements as basic as "declare a war" are at 60%.They said so during the game promotion. A significant chunk of the Civ6 player base never finished a game even among players who play quite a bit.
Most owners clearly don't play, or don't get achievements for some other reason.
But the reason I don't finish games, is if the outcome is obvious. This is for Civ 4, 5, 6. If you can already tell you've won, and you are so far ahead that you'd just need to do busywork to see the end screen, you quit the game.
The constant restarting of eras is probably seeking to address this too, a bit of a leveler with the crisis and end of the era? Again, I'll see how it is on 11th and write a review after I beat a few games.