Sci-Fi vs Fantasy

One thing I've always wondered is why science fiction and fantasy are lumped together as a genre. When I go into a bookstore, the two types of books take up a single section, apart from 'romance,' 'comedy,' and general 'fiction.' Even though the general fiction section doesn't always confine itself to a specific setting (books in that area can be found taking place at any point in history – from prehistoric ages as in The Clan of the Cave Bear to modern day novels, which are legion).

More interestingly, the 'sci-fi/fantasy' section of a bookstore is often seen as the realm of nerds and geeks by many shoppers. Although it is true that non-nerds are less likely to peruse that section, it's not fair to say any of the following:

• Nerds won't go to any other fiction section.
• Non-nerds won't go into that section.
• Nerds will indiscriminately read anything in that section.

But even more interesting is the idea that the two genres are inseparable. The tendency to lump sci-fi and fantasy into a single section is nearly universal, even amongst nerds themselves. Even the cable channel Syfy (formerly the Sci-Fi channel) broadcast fantasy shows with great frequency. This question is relevant now with the release of the new Starfinder roleplaying game. It basically takes the Pathfinder system and uses it for science fiction gaming (though it still contains magic and other fantasy elements).