Well, superheroes are so associated with comics, and comics are associated with children, so I think it shows up less in written fiction and more in web fiction because it falls into the age ghetto. Like the animation age ghetto. However, the internet lets you get away with writing all kinds of stuff. Some people indulge in fanfictions. Others Power Rangers. The Internet Wrestling Community is pretty huge. And, finally, you get people who can finally do some superhero stuff.
Helping it out is that superheroes are very friendly as far as self-insert characters. A person could probably write about their day, then make up stories of their secret superhero life behind a mask. It also helps that there's a significant emphasis on high school and teens, which a number of people seem to enjoy writing about.
I bet some people are attracted to the idealism. Others might enjoy deconstructing it and going all gritty. It's open to a lot of interpretations.
My best guess is that it has something to do with the same reason I like wrestling...big personalities, wild fights, interesting costumes, fun speeches, and some humor. It's just more fun for me. I don't have a problem with the idea that something can be over-the-top and ridiculous, but also be series business. And while some people don't like superhero stories for this, I actually enjoy the fact that you can go all over the place with time travel, epic fantasy, steampunk, magic, mysteries, crime drama, other dimensions, and sci fi alien stories.
Coincidentally, I'm not sure if my lapse in reading Pact is more because it's not superheroes or if I'm just a little burnt out on Wildbow's style. And as someone who frequently deals with people, I find it unrealistic that there are so many rational actors depicted.