You can do that the other way too. Of course WotC has done neither.
As a DM I would rather they give me everything and then I and my group can choose to dial it down (like we have). But everything comes at this a bit differently!
I have tried, over the past few years, to look at the design from this perspective: Which direction is easiest to move away from, but hardest to move toward from somewhere else?
Because as a starting point, "give the GM everything and let them pare back" isn't a bad first pass....but its imperfections show up right quick in many ways. E.g. survival-based challenges are, from reports I've heard from plenty of real people on this forum, pretty difficult to do in a fulfilling way in 5e, because it started from "giving the GM everything" in terms of players having tools to address those problems, such that if you have even like two full-casters, survival almost certainly won't be a problem. (E.g.
goodberry, tiny hut, druidcraft/prestidigitation/thaumaturgy, control flames, etc.) Given that that has made life...rather hard for them, but the alternative is forcing
everyone to jump through hoops just to survive, or to skip early levels? There may not be a clear answer, and so we have to start from the ground up recognizing two opposing interests (folks who
do not want to be burdened with survival, and folks who
want survival to be an interesting, stiff challenge).
Conversely, something like "well-balanced encounter building" is something that is profoundly difficult to just wish out of nothing (just ask any long-time PF1e GM how they feel about building encounters for characters above level 12.) A system that starts out primarily unbalanced is very difficult for each individual GM to beat into shape. A system that
is balanced--balanced well and wisely, not trivally and stupidly, and thus
absolutely the hell not "make everything identical"--is trivial to make unbalanced. Just ignore the rules and do whatever you want. There is no difficulty in making a balanced thing unbalanced. Anyone can take a balanced centrifuge and make it unbalanced--just remove something, or swap something to a new spot. It's quite hard to go from something lopsided to something sufficiently well-balanced that you can be confident spinning it up won't result in problems.
And some things are not to my preference, but
should be the starting point for some stuff. For example, that's why I advocate for "novice levels" (almost certainly by a different name), and have for years and years now. Because that way you have levels specifically designed to give the "zero to slightly-above-zero to (etc., etc., etc.) to maybe just the tiniest bit hero" crowd--who, despite my tongue-in-cheek phrasing, I
really do respect and admire, even if I don't share their tastes--something
made to make them happy, while not forcing the entire rest of the player base to dance to their tune before they're allowed to get to the stuff
they find fun instead. Likewise, simulationist fans generally want (perhaps even need!) well-structured, consistent, logical skill and save DC tables, with clear and effortful boundaries for what can or can't be done etc. Even though most users will want to be more loosey-goosey than that, these players need it, and it's much,
much harder to create those tables from scratch for yourself, than it is to just pretend the tables don't exist and instead wing it by intuition. Such tables are important enough to several simulationism fans I've met that I see them as necessary, even though I personally have zero use for them and find them...contrary to the experiences I've enjoyed most in the D&D space.
If we want a big-tent game, that's where we have to look for our tentpoles. The things that matter enormously to specific interests and which don't matter (or even matter negatively) to others, but which are
hard for the "this matters a lot" crowd to put together
And on some things...you are unfortunately embarked, you have to pick a specific stance. When that happens, you do your best to build options, alternatives, or tools to make the GM's life easier when they go off the beaten path, knowing that your product can't be perfect but is still worthy as long as you gave it your best. Good example here being how one user on here (whom I won't name, but you might guess) advocates for making the game absolutely maximum difficulty as its default setting, and telling GMs to ratchet back from that difficulty level if they feel like it. This was advocated, not because it is a difficult
design thing to make difficult monsters--that's actually trivial and the poster in question recognizes that--but because it's a difficult
social task to get your players to accept being put up against enormous difficulty when they're used to lower-difficulty experiences.
That's something I don't think rules can or should try to fix. The players need to be sold on the GM's choices
by the GM. Trying to force that through with the excuse of "well I'm just running the game as written!" doesn't actually help things and simply will foster more resentment and more problems. Instead, this tells me that GMs need more and better tools for
doing that "sell us on it" thing--advice, processes, and tools to help GMs build and maintain the trust required for experiences like that. Such tools exist for many different interests (that's part of what the X-card and O-card are for, as an example, just not for this specific thing), so there's almost certainly more that can be done on that front. Then the game can set a middle-of-the-road difficulty, balanced challenge, so that it only requires light tweaking to go to "hardcore" challenge or "casual" challenge as the group desires.
