OP here. I have one late thought I wanted to add. I suspect that if you tracked the people who completed 90% of the game, rather than 100%, the stats would jump up considerably. I think there are a lot of people who (like me) get frustrated with the end level or final boss battle, where difficulty often ramps up. After dying 20 times, they say "fuck it" and just watch the end of the game on Youtube.
I did that recently with two games (Plague Tale: Innocence and Bramble: The Mountain King). I enjoyed both games, and I would have been happy to complete them, but the developers chose to ramp the difficulty up a lot in the final level (as developers tend to do), and I found it more frustrating than enjoyable. I got tired of dying over and over again, and so I just quit and watched the ending on Youtube.
I don't think I'm alone in that. I suspect that a lot of people do the same. They don't complete the game 100%, but they get 90% or 95% of the way there. I think if you added those sort of almost-completers to the stats, the numbers would be a lot higher.