I can read most Manga from start to finish. There's a clear start point and a clear end point. It's usually the same author, so it's not a bunch of conflicting ideas about a character. And most of them, emphasis on most are not too long.
Superhero comics have decades of history. And most of their creators are either retired or long dead, so we've had a whole bunch of different authors give a whole bunch of different takes on the character, of varying degrees of quality, there's a whole bunch of different continuities, some of which even cross over with each other, so if I want to understand everything, I need to either read decades of comics, or start at some unclear point, and have a wiki open.
And then there's also the problem of popularity putting heroes in everything, even stories clearly not ideal for them, like Spider-Man is clearly meant for stories like "Oh no! The kid's science fair experiment went way wrong and now the gym teacher is a hippopotamus man!" and Batman is clearly meant for stories involving him investigating and unraveling a crime ring that goes really deep, but both heroes have to be involved in intergalactic conflict due to their popularity.
