The best thing i can compare it to is mountain climbing, their is an inate prestige to climbing certain mountains be it the 48 high peaks of the Adirondacks (relatively short) or the Matterhorn or Everest. And if we built an escalator to the top of every mountain it would cheapen the accomplishment of climbing them, atleast to some.

In a way games with just 1 difficulty are like mountains, Portal doesn't have an easy mode but its also a small hill. Pokemon doesn't have a difficulty mode and it can ba all over the place. But souls likes take pride in being true mountains that take skill and experience to beat, and their are no stairs or escalators so only those truly able to climb it can enjoy the prestige of saying "I did it, i beat darksouls".

And semi related is a valid non-gatekeeping reason. Darksouls difficulty isn't from things you can put on a slider, so much if the difficulty is from very custom tailored elements that you can't simply change the boss's HP and Damage and have difficulty modes that feel right. (Put fallout 4 on hard, it just make enemies bullet sponges which isn't fun. They don't become any smarter, they just waste more time to kill.) So for this reason i understand not wanting difficulty modes if the balance is mainly from factors that don't lend themselves to sliders.