I think the problem with a difficulty slider is that it puts lots of extra burden on the developer. Balancing difficulty is already tricky, and now you have to do it n times? And a newcomer to the series might choose "easy", which is not the intended experience for the average player.
My proposal is an "a la carte" difficulty instead, where you choose what mechanics you want to mess with. You could get unlimited estus or health, give yourself any item or any number of souls, freely adjust your level, etc. Basically cheat codes. That way you really choose your own difficulty. The whole game can be a cakewalk or can just skip one area that's giving you trouble. And honestly that would just be fun for everyone to mess with.
Yes, some games have started doing that and giving them as accessibility features, and I love it. I think it's a great way to do it. Mario Kart 8 and Celeste are two examples that come to my mind which do that.
This is really the answer. I don't know why every (singleplayer) game doesn't do this. I know some players like to complain that it ruins the game, but I can't see how. There will always be the default/designed way to play the game, available for anyone who wants it.
I grew up with games before ubiquitous internet access, where you could edit game files easily. If I wanted to play a game regularly I could, and if I wanted to edit stats, I could. It's just a form of meta agency on gaming. It's about having fun, and I think a lot of purists have lost that, at least when it comes to denying others their way of fun.
My proposal is an "a la carte" difficulty instead, where you choose what mechanics you want to mess with. You could get unlimited estus or health, give yourself any item or any number of souls, freely adjust your level, etc. Basically cheat codes. That way you really choose your own difficulty. The whole game can be a cakewalk or can just skip one area that's giving you trouble. And honestly that would just be fun for everyone to mess with.