Dune in particular was so dominated by mysticism and
    politics that I think it would have worked better as
    fantasy set in ancient Egypt.

Scifi requires you to invest a lot of energy in the read, yet a lot of it is rubbish so the ratios are wrong.

Part of the reason I like Philip K Dick is that for the sort of fiction he writes, science fiction often is the best medium for communicating. And, he doesn't mind producing books where part of the story doesn't even make sense in a rational sense, so long as it's progressing the message.

In _Time Out of Joint_ the game world is implausible. The messages on paper doesn't make sense. The brainwashed characters seems like a big thing to be taken for granted. The edge of the world doesn't add up. However, the protagonist's attempts to come to terms with the world he's in is believable.

Dick writes fiction as I seek to hack. "Am I solving the problem I care about?"

The benefit of the lack of context is that scifi can serve as a literary equivalent of a rapid development environment. The author is able to summon technology or magic in order to create settings that exercise the characters. _Ubik_ uses not-subtle examples of this in order to explore the ideas it cares about - precognition and a variation of brain-in-the-vat.

Unfortunately, the mainstream culture of fiction and fantasy epic (Lord of the Rings, Dune) actively fights this flexibility. They try to construct castles in the air - worlds that take significant energy to build and comprehend, and which thereby become limited by the same barriers that affect tales set in real settings. Having done all this work, many then tell stories about princesses, knights and dragons. Dune managed to rise some way out of that.