Why most DM advice is useless

DMing is first and foremost about running the game.

The best advise I have about becoming a great GM is: Just do it. Listen to your players. Reflect upon what worked and what did not. 

"DM Acting" is licensed under CC-BY 4.0. 

I think written advise about how to run the game is largely useless, because GMing is not a knowledge skill, it is a skill acquired through experience. You have no time to consciously think about what you should do while you are doing it, it is like driving a car. Once you get it, you can recognise true advise, but you do not need it any more. 

That is why watching a great GM or playing under one is the best way to learn it. The best advise I can give about running the game is: watch Chris Perkins at PAX do it. The guy is a genius. It is blissfully entertaining, and you can experience it. You could watch Critical Role, but I think Mercer is not quite as good as Perkins.

DMing also involves preparing the session, so you will be able to effectively run it.

Advise for preparing is probably the area of the game where blogs and DM advise are most useful. There are many resources for this: Youtube channels like Dungeon Dudes, blogs like DMDavid, Mike Shea's Sly Flourish Lazy DM tricks, or The DM Experience where Chris Perkins wrote about it at length. I like Perkins' recommendation to start each session with a short recap (although I don't do it), or Mike Shea's to make a brief list of secrets that the PCs could uncover (although I don't do that either). 

DMing lastly is about creating a world and adventures.

The best advise I can give about creating is: don't, unless you enjoy doing it for its own sake (I do). You do not need to do it. There are published settings, campaigns and adventures, written by professional game designers. Just use those. This might feel a bit like DM on training wheels, but the best of them are as good as anything you would be likely to come up with. All you need is to prep them so you know where to find stuff, and maybe to tweak for your group.

There is all the advise in the DMG trying to explain how to build worlds and dungeons. All I really use from the 5e DMD are chapters 6-9: the reference sections about downtime, magic items, rules options and monster creation, and the latter only because I convert old modules into 5e for play. 

So why is this in the DMG, when it would be in the publishers commercial interest to direct you to their other offerings? I believe because it matters to the designers, and they want to share. Or, if you are more cynical, because the game is marketed as a creative one. Or it is tradition. Back in OD&D and 1e Gary Gygax expected each DM to build their own world and dungeon, so he had advise and a toolkit on how to do it.

If you find writing adventures is fun, I learned more about good adventure design and presentation from the dungeon magazine reviews by Bryce Lynch at tenfootpole than from any other source. Read them, or you can get the cliff notes version here.