Why Don't Authors Switch Genre?

Graham Edwards brings up a point I've wondered myself many, many times:

Movie directors – if they so desire – are allowed to tackle a range of genres. In the course of his career, Steven Spielberg's bounces from The Sugarland Express to ET to Schindler's List to Saving Private Ryan and beyond. Danny Boyle's practically made a career of picking a different genre for every film he makes.

But what about writers?

Okay, let's pick a few big names. Terry Pratchett? Oh yes, he writes fantasy – the funny kind to be precise. James Patterson goes for crime thrillers. And that nice Philippa Gregory does historical. Their names might just as well be genre tags. The fact that they've all have had work published outside their respective pigeonholes has no effect [.... ]

So what's the problem here? Is it the authors who get stuck in a rut or the readers who put them there? Is it the publishers needing handy and reliable blurb, or the merchants needing books that fit under their equally handy and reliable categories? Is it all, god forbid, driven by focus groups?

— Why don't authors switch genre? | Graham Edwards Online.

He does mention several authors who manage to write one type of book under one name, and another type under a pseudonym; he doesn't mention any of the rare few who, like John Shirley, have managed to build a name for themselves in one genre and then switch gears and successfully build a new reputation in another.

I wonder about this phenomenon myself, quite a lot.  I've definitely tried to position myself as "a horror writer," and will probably continue to do so — but I do come up with ideas for stories and novels (and webseries) that are definitely science fiction, and not horror at all.  Should I adopt a pseudonym for those?  Would readers who enjoy my horror work be confused or turned off by finding work from me outside of the genre?

I also wonder if the current self-publishing revolution will change this at all.  Surely without publishers and agents telling them they have to stick to a consistent brand identity, more writers will start to choose to play in other sandboxes whenever the feel like it.  The question is whether or not readers will follow them there.