Sci-Fi not Fantasy

I've never understood why booksellers group Science Fiction and Fantasy together, I just can't see the relationship. In fact, why do they split it out of ordinary fiction? According to Wikipedia, Science Fiction is defined as:

...a broad genre of fiction that often takes place in the future and usually involves speculations based on current science.

While Fantasy is defined as:

...a genre of art that uses magic and other supernatural forms as a primary element of plot, theme, or setting.

Both articles do note, however, that the genres are generally lumped into a broader category called Speculative Fiction, which means it generally includes science fiction, fantasy, horror fiction, supernatural fiction, alternate history, and magic realism.

For me, the definition of those two, and many of the other genres that make up speculative fiction, are so different I don't understand why they are split out. Likewise I find it odd as to why they are split are usually kept in a totally separate section of bookshops, why not with the rest of the novels? It always seems to me that Sci-Fi and Fantasy as considered lesser genres. Not all of the genres of Speculative Fiction are treated in this way, however. You'll find Robert Harris' novels in the ordinary fiction section, though they're typically described as historical fiction. Likewise, novels by Thomas Harris feature there too, but walk the line between Thriller and Horror.

So, booksellers of the world, lets get some things straight. Science Fiction and Fantasy are not the same thing, far from it (they're practically opposites) and neither genre should be singled out into a special section, include them with all your other fiction, they're just as good and just as worthy.