I think my issues with the 5e dmg can be epitomized by the section on experience points. Here's a typical problem: a new DM wants to award experience points for the social and exploration pillars of play. "How should I do that?" they ask the internet. Well, there's a section in the DMG all about awarding experience points! Here's what it says on that topic (p. 261)
Are those sentences of advice helpful? Are they specific to 5e in any way? If our hypothetical new dm just decided to make up a system of how to award non combat xp, what are the chances it would look exactly like this, but be better and more detailed?
Also why did our new dm not find these "rules" to begin with? Let's see where they are located in the book: they are in the chapter "running the game," which for some reason is chapter 8, near the end of the book. The section on awarding xp--a central facet of the DMs role--is located after rules for chases, siege equipment, diseases, poisons, and madness. Those are all not only not essential, but they very well may never come up in most campaigns.
The thing that probably frustrates me most about the dmg (and I realize this is an odd thing to be annoyed about) is the layout. Let's look at the actual page:
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WTH is going on here? Instead of just fitting all the "rules" for experience points onto one page, they split it over two pages (the second page is half text and half art). Moreover, the section is actually split by the "madness" tables, which are much more prominent on the page. The 5e DMG is full of layout like this. It's like they just copied and pasted the text into their layout software and called it a day. It might not seem like a big deal, but it really affects the usability of the book, and makes it so that rules and advice (which, imo, is often half-baked to begin with) gets lost on the page.
So it might not be that people are being "lazy" in not reading the dmg. Rather it's a hard book to read and reference, the text is difficult to quickly parse, information is scattered and poorly organized, and pieces of good advice or helpful rules are lost amid all of the anodyne suggestions.
/rant