An important note: This has less to do with difficulty than it does with general player behavior.
The director for Hitman: Absolution, Tore Blystad, has told OPM the following:
20 per cent of the players will see the last level of the game. It’s horrible to know. It makes the people working on it really, really sad.
We are using metrics a lot more now than we did, for good and for bad. The general player will probably never even finish the game, which is very sad. Or they might only play through it once, but the game is built for the people who want to go back through every single level and get all the stuff out of it. It’s built to last, rather than be a one-off experience.
I can see how people can complain about how games have become too easy and that making games difficult are a detriment to players finishing it. That is a valid argument, though most of the issues with difficulty have been solved by giving players the choices between various settings.
There are many other reasons why people don't finish games, like getting bored or distracted, being distracted by other games on their queue, and thinking about the time investment involved and concluding that it's just not worth the effort.
I usually finish all of my games (about 60-70% of them), or I just don't start them in the first place. What about you?