That's actually part of the answer. It took decades for enough fans-of-the-genre to get enough clout in Hollywood to get those comic books put into serious movies.
-The fans didn't drive the "superhero boom". They were the target audience, yes- the kids reading comics in the 80s and 90s are old enough to be their own demographic now.
The thing was, nobody took superhero works seriously for years- decades, really. We had things like the crappy Captain America movie and even crappier Spider-Man TV series back in the late 70s, and the not-really-true-to-the-character Batmans of the 90s.
It finally took the success of the first X-Men film in 2000 to turn the tide. It was a big-budget, star-laden film that, thanks to a director who was a true enthusiast of the source material, stayed, well, true to the source material. (At least, given cinematic limitations of time.)
That showed Hollywood in general that a "comic book movie" could succeed- not just succeed, but become a blockbuster. And they realized that hey, there's like fifty years worth of comics out there, spanning hundreds of titles and hundreds of well-known, even iconic characters.
Marvel tried to cash in by selling off the rights to Spidey, the Fantastic Four, and some others (some of which happened long before the X-men movie came out) before realizing that those other studios were basically destroying the franchise- IE, the crappy third Spider Man, the almost-as-crappy third X-Men- that actually killed off a prime character just because the director personally didn't like him! - the crappy first Hulk, the crappy first Fantastic Four, and so on.
However, thanks to an influx of cash from the successful ones (the two X-Mens, the first two Spideys) Marvel decided to launch their own studio- after all, who better to handle their characters than their creators?
That led to the first of the "Marvel Cinematic Universe" movies, Iron Man, which knocked it out of the park. Then, brilliant planning and more brilliant movies and they've got a franchise going, which they then sold to Disney for approximately All The Money.
Anyway, there's no money being made in web comics (that I know of). Therefore, Hollywood probably won't be interested.
-Yes and no. Unfortunately, today, movie makers are a bit gun-shy, especially on the big-ticket movies ($100M budgets and up.) And so want the "assurance" of a ready-made, built-in audience to target.
But that doesn't mean you can only make movies of things people already know about. There were no "cyborg police officer" fan clubs before RoboCop came out, there was no long book series on the Titanic before that movie was filmed, and there was no big resurgence in Egyptology gripping the nation before Stargate was made.
Hell, I'm old enough to remember when action movies were written specifically for an actor: Commando, Bloodsport, Demolition Man, Predator, Cyborg, Lethal Weapon, ad nauseum.