Exactly, Marvel makes its money on the perception from its fans that they're more down to earth, so they've focused on a specific group of characters that fit that mold while marginalizing the ones who don't. Hence, Thor is dead, the Silver Surfer hasn't had a decent ongoing in years, Dr. Strange hasn't had a decent ongoing in even longer, and the only time their really big, outlandish villains like Fin Fang Foom ever show up anymore is to be parodied in the likes of Nextwave. DC, on the other hand, has found that a broader range of characters and situations sell well for them, so they've focused on inclusiveness. Creators can pull from practically everywhere--mythology, cosmic space operas, sci-fi, gritty crime drama, basic superhero action, and more--to tell their stories, resulting in comics like Firestorm and Green Lantern Corps running right alongside the political intrigue of Checkmate or street-level action of the Bat-family's titles. Regardless of what sells, however, both publishers have the same things, ranging from the grit of the Punisher or Batman to the far-flung space adventures of Green Lantern or the Silver Surfer. Neither is more down to earth than the other because these things exist in both; people have simply come to perceive Marvel as being down to earth and Marvel has accomodated by cutting the other stuff out of the picture--relatively recently, I might add. Fifteen or twenty years ago, Thor and the Silver Surfer sold almost as well as Spider-Man and the X-Men.
And I know that the FF is a family, but the wild adventures is every bit an integral part of their comics as the family aspect. The FF without crazy sci-fi is like Dr. Strange without magic; you can't have one without the other. Hell, Kirby partially defined his super-sci-fi, colossal machine style on the FF. Reed develops patent-worthy inventions before breakfast, they ride around in a detachable vehicle called "the Fantasticar," and they have interdimensional portals and time machines strewn about their house. Oh, their giant, skyscraper-with-a-"4"-on-top house, I might add. The FF are science fiction as much as they are a family. That's why the best runs--Waid's, Lee's, Byrne's--have all incorporated a good mix of the two.