Bah... assuming there really is a shortage which I wouldn't be so sure of, I'd blame it on the player base.
I'm still here DMing the game a bunch of times a year despite too many other interests and hobbies. But I don't want to run the game for anyone else than family, friends and coworkers. Everybody else, I don't want to play with anymore. I don't want the chance of having ruleslawyers, powergamers, people who want to fix "broken" rules, who treat the DM as their enemy, or whose real interest is just buying/collecting stuff. Chances are that someone among my own friends and workmates might be those kind of people, but when they know me already before we play, they're not going to let that side of them come up.
Conclusion: aspiring DMs should take advice from "old school" DMs...just go for it. If you're with friends, they'll love the fact you're trying hard, and you'll get more confident and better as you go. Don't look for your inspiration by mimicking other groups.
Best advice around here. I learned to be a decent DM just by doing it, although I was a player before that so I kind of had a model to refer to, but I know that my previous DM also learned it by doing it.
And being decent is all you really need. All this idea that DMing must be a competitive sport is ridiculous, but then it is also the same bad culture that puts most people off doing something they would like in life, because the message is either you are exceptional or don't bother.
And as I said, playing with people you know indeed goes a long way making DMing a lot more comfortable.
Here's the thing:
Do I want WotC to release a product that actually teaches DMing?
Absolutely.
Do I actually know what that product should look like?
No.
Maybe that's a useful clue... practically every hobby is infested by the idea that your problems will be solved by buying something more. "How can I get better at playing guitar? Buy a faster guitar!" (recurring joke on music forums). There is a whole industry around each hobby, the sole purpose of which is to get your money. That isn't a problem per se, but it becomes a problem when you don't realize it is also taking your time.
Putting aside the general anxiety that a new DM might face, the books aren't making it easy for them.
The rules for creating monsters are a bit obtuse, and don't really give a fair try for measuring non-DPR contributions to combat.
Encounter building is likewise nebulous.
Most of the pre-written adventures require heavy amounts of re-writing and prepwork.
I agree that despite 48 years and countless editions, designers still haven't managed to arrange books in a very good way, and the text is always more oriented towards technicalities instead of usability. Unfortunately I have little expectations that the next iteration will see any improvements.
As for the current edition, I believe it would go a long way if they just embraced the idea that any edition can be a toolkit. Instead of presenting the game as a set of 100 mandatory things, it could easily be presented as only 5, and the rest "to be used later, when you feel ready". But mandatory things sell better than optional ones, because they give the impression that the game you bought is "bigger", so they are now increasing them to 110.