First- all this material that most people are clamoring for *already exists*, and it's an easy-ish convert.
It's not that the material doesn't exist or how hard it is to convert. There are a few issues at play...
1) Aesthetics. It just feels better to have everything as a "set". I could see a strong argument for publishing a no-mechanics version of a setting's fluff, with a gazetteer (print or PDF) for the mechanical stuff. But... that would require a reprinting, anyway, at this point. This is a fairly weak argument, but it's probably the one that
truly motivates most folks who want an update.
2) Availability. Sure, some of the stuff is available via Drive Thru. The desirability of PDFs is very much subject to personal taste. I don't mind (prefer, actually) having PDFs of rarely used books, but much prefer anything "core", including the setting book, to be hard-cover. Also, no one is going just stumble upon, say, Eberron by accident, which brings us to:
3) Momentum. Only publishing content for the Realms is a self-fulfilling prophecy. As someone who likes Eberron, is warm to Dark Sun and Greyhawk, but absolutely does not like the Realms, it's very frustrating that the
only setting that a new player
can get exposure to on the store shelves is the Realms. Blech. Sure, the Realms don't have their own setting book, either, but they
are getting updates and "screen time" by default.
That said, I understand where you are coming from- especially regarding issues like, say, classes in Eberron.
This one is a bit different than the above. There really isn't a palatable substitute other than rolling my own, for which I simply don't have time.