Difficulty settings are not so much black sheep of video games, as they are profoundly neon-blue ones, grazing noticeably among the flock of television, cinema, books and so on. No other medium invites the viewer to adjust the experience they will soon delve into according to their level of skill, or their definition of fun and challenge.
The notion of temporarily denying your audience a portion of your work due to their inability to progress is enough to send directors and authors of other industries into fits. It's madness, and counter-intuitive for most media to do so. Imagine movies that regularly give the viewer a pop-quiz assessing their knowledge of preceding events.
Incidentally, that exact approach is one of the defining characteristics of video games and deeply rooted in the eclectic elements that have made the medium so popular.
A middle finger from a final boss is, when employed effectively, just another brick laid upon the player's gas pedal; one more thing pushing them forward. This builds a unique drive to beat that boss, reach that checkpoint or otherwise improve, not found anywhere but in games.
But all those triumphant and hard-earned victories do not come without the worst of defeats. Simply dying would be mercy; far worse is the reluctant admission that you would rather progress on Medium than continue to bite the Hard bullet.
Sometimes it's your sanity or the next level, and sometimes, you just need to wuss out...
Note: Some of these entries are final bosses; read their slight spoilers at your own risk.