[...]there's far too much worthless junk clogging up the limited science-fiction/fantasy/horror (why is horror lumped in with those two ?)[...]
Again, this (at least, as a full discussion) is a subject for another thread but the (relatively... hey, Ace, you know
me) short answer is that all three emerged from the same eighteenth-century reaction to the prevailing literary views of the time. Admittedly, "horror" (or terror, as it was then called) is the oldest of the three, with the advent of the Gothic novels and shorter tales; but even within this limited aspect, there were plenty of elements of outright fantasy (see, for instance, "Sir Bertram: a Fragment") and, with the advent of such things as Frankenstein, not to mention its largely forgotten predecessors, or Poe (who frequently used themes and tropes we would consider early science fiction, both for horrific and humorous purposes -- or sometimes, as in "Metzengerstein", both simultaneously), the "scientific romance" entered the picture as a form of its own... yet still in many ways inextricably tied to these slightly older genres.
Those origins have, by and large, stuck with the three forms of fantastic literature ever since, both in the critical and general public mind (as well as that of the people in marketing); this is even more to be expected when one sees how often elements any one of them enter into the composition of the other two. Examples: King's "The Jaunt",
The Stand; several of the tales in Barker's
Books of Blood, or his
Damnation Game,
Imajica; Tolkien's writings, which often have a fair degree of the terror tale to them; Robert E. Howard's fantasies, horror tales, and interplanetary novel,
Almuric; Moorcock's blending of the three in a considerable portion of his work; Andre Norton's
Witch World series; Leiber's Fafhrd and Gray Mouser tales; Lovecraft; Dunsany; Ballard; Ellison; Tanith Lee; even Heinlein, with various of his stories and novels... the list goes on and on.