One consideration is that largely, what we consider the GM, and the power afforded to the GM, has the most to do with the responsibility placed on the GM's shoulders. If players accept that a GM is responsible for following the rules, then a set of powers enabling them to bend, break, and redesign the rules for the purposes of some other goal becomes needless. The more broadly defined their goals are (say, to avert bad feeling), the more necessarily wide their powers become-- this can be restricted in such a theoretical constitution, but the possible discrepancy between responsibility and powers, is likely a source of pushback.
Your point regarding the specific engineering of each game as a tuning of the GM role is a good one, although there is a part of me unhappy with it, because the setting of expectations as happening only in the game feels like a perfectly spherical cow in some ways-- there's a lot of tension when a rulebook stakes a strong position because players often don't see their expectations as being on the table for the book to alter, so it's a non-starter. So you see this natural inclination to reload the GM with authority, and the driving responsibility is "present the experience we wanted it to be, not the experience the book is evangelizing." This can even be true for the book itself, which will essentially infuse the GM with the responsibility of carrying forward its evangelism "this game doesn't suck, your game sucks."
So to get rid of that authority, you have to get rid of the responsibility, and it kinda seems like things have been going in the opposite direction-- the emphasis of the OC thing that's been treated as co-mobid with 'Neotrad' design is about exerting that kind of control on the game in the first place, for a player-desire centric experience.