The FF were never meant to be regarded as superheroes. They've always been adventurers and explorers of the unknown--that's what sets them apart in the superheroic community.
Indeed. Remember, when FF began, superheroes as a genre hadn't quite made a comeback at Marvel yet. The first couple of issues were more along the lines of horror comics, with the FF in plainclothes and battling monsters and alien invaders. It wasn't until the third issue that they adopted costumes and started fighting supervillains, but it was always in their own characteristic way that defied superhero tropes -- for instance, they never had secret identities.
Within the Marvel Universe, the FF are more like the royal family than a crimefighting team. When other heroes need a scientific consult, they visit Reed Richards. When Earth faces a diplomatic crisis involving aliens or Atlanteans or Inhumans, Sue is the ambassador and peacekeeper. And a large part of the reason that other Marvel heroes have to take care of so many alien invasions and other dangerous situations themselves is because the FF spend so much time off Earth or outside our dimension, exploring other realms. So really, if the FF are just portrayed like any other superheroes, it's not at all accurate to the comics.
Besides, what's made the modern superhero genre so engaging is that it's so many different genres in one. Not every superhero movie is about crimefighting. The Iron Man films are techno-thrillers.
Thor is supernatural fantasy with a Shakespearean flavor.
Captain America was a WWII movie, and the sequel will be a gritty, verite-styled spy thriller. The X-Men films are political and social thrillers, allegories about bigotry, and in recent years they've become historical fiction as well. And
Guardians of the Galaxy is going to be a big space opera sort of thing, evidently. So why couldn't FF also be a big adventure/sci-fi film about exploring the Negative Zone or the Microverse or traveling in time or whatever?