Because you're not giving actual specifics and responding in platitudes? The DMG is not going to tell you specifically what to do because that's not it's goal. The game gives examples, gives options, but it's always clear that people need to figure out what works best for them.
There will always be a bit of trial and error when figuring out how to DM. You are asking too much if you think there is any way a book could boil down the choices into what will work for a group, but they do give guidance. Take the Roll of the Dice section Rolling with it:
Relying on dice also gives the players the sense that anything is possible. Sure, it might seem unlikely that the party’s halfling can leap on the ogre’s back, pull a sack over its head, and then dive to safety, but with a lucky enough roll it just might work.
A drawback of this approach is that roleplaying can diminish if players feel that their die rolls, rather than their decisions and characterizations, always determine success.
It gives very clear and concise benefits and drawbacks. How is that
not actual advice? It doesn't matter for this whether you're doing an urban mystery or a dungeon crawl, the benefits and drawbacks will be the same. What else could they do? Have a quiz "Answer these 20 questions to see what kind of DM you are?" Because that's just not practical, you may as well tell them what's best based on their astrological sign.
For many things the only way to learn is through experience. I can read all the books I want on how to drive, and some may be useful, but ultimately I've got to get behind the wheel.