The simple fact is everything else about the game is so good that either ignoring the lack of difficulty or adding your own challenge means you get to experience a great game. My ghost playthrough was very fun and often challenging, and the game is designed for it, it's not like you're larping.
I don't know how you can say that with a straight face. The mission design in Dishonored is atrocious. You never have to solve any puzzles or problems, you never have to even think. You just wait a minute to see people's routines, then you blink in and kill them all. In storytelling, they say that a story is only as good as the villain. In Dishonored, the "villain" is massively outclassed. You are a practically unkillable superhero (actually better than that, because unlike most superheroes you have multiple insane superpowers) and your opponents are a sprinkling of human guards with poor AI. Even in the final level, where the boss of the game is literally hiding away FROM YOU in his so-called "fortress," your only opponents are basically the same outclassed human guards who wander around solo in a sparsely populated compound with at least two completely open entrances. It's as if the place was specifically designed to be infiltrated by someone just like you! Oh wait a second, now. To get to the final boss in that level, you have to kill, what, 10 guys? At most? Nothing is locked down, nothing requires thought, you don't have to even figure out a specific sequence of exploration. Just watch, blink, kill. Achievement unlocked.