Hiya!
That's very strange indeed. This is something that the DMG spells out explicitly?
Without any guidance, it's an ignorant risk and thus a false flexibility of the game.
First, no. The DMG doesn't "spell it out explicitly". It shouldn't have to. People, especially DM's, are expected to use their grey matter and experience to make the game their own. I mean, if someone is handed 4d6, a piece of paper, and a pencil, and then told to take his three friends and come up with a dice-game that can be played around a campfire...they will. A game will be created. It will suck. Then it will suck less. Then it will be ok. Then it might even be fun. Then it could get to be ridiculously fun. Should those people have just immediately said "Nope. Can't do it. Without explicit instructions of what to do or not to do, it's impossible"? Well, same thing here. "Here's the core rules. Play it. Change things you don't like. Add things you want and remove things you dislike".
Second, RPG'ing isn't a "normal" game. RPG's, by default, are customizable to the individual group. And with them, the DM has a special role to play. He/She is the "designer of the game rules and ultimate authority on how they are applied". In Monopoly someone can point to a rule and say "You are wrong"...and there's not much anyone can do to dispute it. It can be changed, but EVERYONE has a veto. With RPG's and the DM...not so much. The DM can veto something...players can not. They can accept it, or walk away.
The point is that with DM'ing a campaign of 5e, the DM learns as he goes. He gets better. He learns tricks and techniques that fit his style and he learns to adapt the rules to tell the type of stories he favours. There is no magic bullet for "insta-fixing" anything. There just isn't. It's also impossible to include rules suggestions or guidelines for
every little thing. Just search the net and you will see a myriad of DM advice pages, books, podcasts, video channels, blogs, etc. One DM will say "Never do X!", and another will say "Always do X!". Both DM's are correct. Both DM's have "balanced" their system, and both DM's guidelines on how to run a game using X are doing it right.
So, as you can see, it is impossible to lay down an absolute this/that/theotherthing with regards to "balancing" a campaign. You can fiddle with numbers all you want, but the bottom line is that your game will be different from another DM's game...even if you are both using the same exact rules and guidelines. Because with RPG's and DM'ing, personal taste, style and experience make a MUCH bigger impact on how good/balanced a game is than some formula that says "If you add + items, you have to do X, Y Z to counteract it".
No getting around it. You want to know how to balance your game? Keep DM'ing, take notes, and be consistent. You'll get there.
^_^
Paul L. Ming