Fantasy sells. Bookstores and theaters are brimming with works of fantasy, be they Harry Potter, C.S. Lewis, Dragonology, Eragon, or Lord of the Rings. Science Fiction sales are in decline, while Fantasy sales are shooting through the roof.
C.S. Lewis (Chronicles of Narnia) and J.R.R. Tolkien (Lord of the Rings) were English faculty at Oxford. Robert Jordan (Wheel of Time) has a BS in Physics. J.K. Rowling (Harry Potter) has a BA in French. Christopher Paolini (Eragon) has no higher education as of yet.
These fantasy writers have inspired countless fans to stand in long lines at movie theaters and books stores sporting capes, light-sabers, and elf-ears, while endlessly debating which of their favorite trilogies is superior based on purely subjective criteria.
In contrast, David Brin (Uplift) has a Ph.D. in Space Science. Stanislaw Lem (Solaris) could not attain his medical degree because he refused to accept Lysenkoism, but did work as a scientific researcher. Dr. Isaac Asimov (Foundation) was a professor of biochemistry, Vice President of Mensa International, president of the American Humanist Association, and wrote hundreds of books on science, politics, and human improvability.
These science fiction authors have accurately predicted the future from cell phones to the Internet. They have contributed to the human race’s collective body of knowledge, and they have inspired countless others to do the same.
Science Fiction fans are intellectually engaged with their subject matter, taking the speculation beyond what is presented, and internalizing its vision to inspire their own accomplishments and contributions to society. Science Fiction walks alongside civilization, evolving and growing in potential as we grow and evolve as a society and a species.
Fantasy books use printing presses and desktop publishing software to glorify times when most people were illiterate. Fantasy movies use computer animation and special effects technologies to let people escape to worlds without films and special effects. Fantasy video games whisk players away to realms devoid of computers. Fantasy wants to delude us into thinking things were better, more exciting and morally clear in mythical ancient times without electricity, running water, toothpaste, toiletpaper, fast food, equal rights, aspirin, diet soft drinks, or any of the other myriad conveniences of modern life that science has betowed upon us. All fantasy fans have to look forward to is bigger swords, flashier magic, and more gruesome monsters. Fantasy is an intellectual dead end.
While fantasy broods on an overly idealized dramatization of the past, science fiction looks upwards and outwards to the future. As L. Ron Hubbard said:
[Science Fiction] is the herald of possibility. It is the plea that someone should work on the future. Yet it is not prophecy. It is the dream that precedes the dawn when the inventor or scientist awakens and goes to his books or his lab saying, ‘I wonder whether I could make that dream come true in the world of real science.’ Science Fiction argues that the best times lay ahead of us, but only if we make them happen.
Man does not really want to explore space, he only wants "to extend the boundaries of Earth" through space, finding worlds that are like the Sahara, the North Pole, the Amazon basin... Science fiction creates worlds that are in reality only distorted images of already known "worlds"- another Sahara, another Antarctica. And this partiality does not represent just the common SF problem of presenting something no one has experienced, or using unimaginative methods of alien-creation. The difficulty is more psychological and sociological. As Snow puts it:
We think of ourselves as the Knights of the Holy Contact. This is another lie. We are only seeking Man. We have no need of other worlds. We need mirrors. We don't know what to do with other worlds. A single world, our own, suffices us; but we can't accept what it is. We are searching for an ideal image of our own world... (chap. 6)
In other words, we want a perfected and idealized version of the old Earth (Anderson's habitable frontier worlds are excellent examples.) We do not want to experience the alien, the supposed justification of space travel; we want to extend Earth across the universe- "cosmization" openly admitted, an imperialism of intellect. In seeking out other worlds, we are really only seeking a better version of ourselves, because the true version we are trying to escape, as we are trying to escape the true Earth.
We assume that human complexity will somehow evaporate when we all get out there and sail through space... Our longing for alien worlds is only an attempt to escape guilt, to escape awareness of the self-imposed ravages of our planet; to avoid facing the contradictions of SF's special group and instead to spread its single-minded influence, unquestioned, among the stars.
tl:dr; They're both uncreative tripe. Not that 'normal' fiction is any better either, but nor does it pretend to be.
I don't know what's so bad about wanting a perfected and idealized version of our Earth, or our lives. Fantasy and Sci Fi can both give people inspiration to do that. Fiction can be a great medium to teach people to aspire to greater virtues.
SciFi and Fantasy are both speculative fiction, and are at their best when exposing new perspectives to an audience. Realistic fiction more often just reflects our own beleifs; forcing us to face them.
Of course, these two converge in fantastic parts of realism and realistic parts of fantasy. There is no strict boundry, just personal preference in how the world is presented.
(I say that a book about someone escaping a town held back in time into an unexplained and non-mundane modern world is fantasy.)
Good fantasy and good SF are supposed to be a way to take perspective on our own world and make us think of its whereabouts.
With an added entertainment factor that depends on your tastes.
SciFi is probably more relevant for philosophy as it often poses as an extension of our own world rather than a "parallel" reality.