To be clear, I'm not saying it is primarily for beginners. All I'm saying is I understand why people think it is/should be. And if that's a common misconception, it's probably worth addressing.
I am going to point this out again.
Point 1
5e is the most popular version of D&D in history.
It's the most popular TTRPG in history.
While I can't pull the numbers, I would wager very good money that 5e has introduced more new people to the hobby than any other TTRPG in history.
So, something good is happening?
Point 2
Most people who come to D&D aren't completely new.
They have either learned to play by joining another group, or learned to play by watching other people play extensively.
For the people who come in completely, 100% new, there are things called "Starter Sets," which are designed for completely new people and are sold through mass-market retailers.
Point 3
The "core three" books are, and will remain, that way because we've always had that. A PHB, a DMG, and an MM. Now, someone
could order a MM from an online retailer, thinking it's a book about playing as a monster (it's a manual, after all, not a compendium), but they aren't going to change the name.
In addition, given the easy availability of video and on-line resources, using a non-interactive text to teach people how to DM seems like it is not the best way to do so- which is why WoTC is using videos in conjunction with the Starter Set (for example).
Point 4
The complaints you are seeing are not, for the most part, from people that are playing and loving 5e. If you look closely, you might notice that it is mostly people who (1) don't play D&D (or 5e); (2) don't particularly care for D&D (or 5e); or (3) are very keen on telling people to play games in a certain way.
When you combine these issues, I think you understand the skepticism. Can the DMG be better and more accessible? Certainly. But I would take the complaints on this forum with a few pounds of salt.