It's a section of advice targeted at an audience other than the one that needs to be the one getting the information in order to meet any useful goal. The PHB & DMG are written for the same game but need to have information that targets the side of a gm screen that most effectively supports the game. Pert of that involves putting information in the book that needs it. Unfortunately this is a case where the GM starts out fighting uphill & against the current because of how the PHB so deeply pushes "you be you [don't worry about the unmentioned GM & rest of table]" to players.
No, they are targeting the one they need to reach a useful goal. Let's say that they put all this in the PHB, and the player's show up expecting a sessions zero... and get a session 1 because the DM planned a session 1 instead. Is there going to be an effective sessions zero? Not likely. The DM schedules the sessions, so they have to schedule session 0.
And, again, maybe you haven't read the PHB in a while, but the PHB is packed. Let's call it 300 pages. The PHB spends over the first third of the book presenting the player options to make their character. Race, Class, Background. approximately 125 pages. There is then another over one-third spent on the rules of the game. How do ability scores work, spells, how does combat work, how does this sub-system work. It is from 170 to 290. So, that is 240 pages out of a 300 page book devoted to purely "how to make a character and how to play the game" Rather important. The remaining 60 pages? Equipment for your character and feats for your character.
Sure, maybe there are things they could have taken out to transplant this entire section in, but here's the point. There isn't exactly a lot of room for a philosophical discussion on the relationship between the player, the other players, and the GM. The game figures, like most games, that you are playing with a group of people who want to be there, and you can all figure out how to cooperate.
In this case the TCoE session zero section
Wait wait wait, you're quoting Tasha's? Tasha's, the book packed to the brim with player options? I'm in a different country right now and don't have a digital copy of Tasha's, but it is like... 80% player content. So why are you griping that this isn't player facing? Players use Tasha FAR more than DMs do. That's about as player facing as you can get.
is targeted at some vanishingly small segment of the community made up of players who really want to have a session zero but can't because the GM doesn't believe it's a thing if wotc didn't publish it. That crumb of a slice of a slice is probably not large enough to justify having nearly three full pages devoted to it making it a pretty big failure to support players who don't know what it is. The whole quest in 5e's design was simplicity at all costs no matter the cost, supposedly to make it easy for new players... A session zero section that dumps the entire task of getting the bulk of the table up to speed on what is assumed by the text to be the freshest & noobiest of newbie DMs who don't even know what a session zero is fails once more in a category that results in actively making things needlessly difficult for the only remaining person at the table not directly failed by the first failure.
I'm honestly having trouble even parsing this. The section is a failure because you don't think it is player facing (which, turns out it is) and that it is for only for GMs who don't believe it is real (it isn't) and expecting the DM to lead a session of the game where they are expected to lead game sessions is a failure because it puts too much pressure on the DM to cooperate with a group of people they are expected to cooperate with?
My man, if you as a DM can't sit and ask some players "okay, what are you playing? Okay, how about you?" even if they aren't in the same building at the time, you are going to have far far larger problems in the future.
Session zero advice needs to be in the PHB not DMG & written towards getting the players ready to constructively participate in session zero rather than writing it to dump the task of doing that on a DM who doesn't even know what a session zero is. The GM facing section in the DMG need not even exist simply because there should be general sections about the very ongoing GM side collaboration things like working with players on X.
I really don't know what to say, considering you cut the vast majority of my response to you, but frankly, you're just wrong. You seem to think it not being in the PHB somehow means that you will get belligerent players who refuse to work together, but that if you had it in the PHB then magically they would be model teammates. It doesn't work that way.
And the only thing the players have to be willing to do to "constructively" have a session zero is 1) Answer questions 2) Be willing to work together with the DM and the other players. Incidentally, the same things they need to be willing to do to play a game of DnD. And the information in Tasha's is also player facing, so that's like... your entire complaint gone.
