This is also why you sometimes see mechanics based on interpersonal drama and self-image, instead of concrete powerscaling and skills. The rules aren't really about making a toolbox to construct challenges (GM) or solve tactical problems (players), or on simulating game-reality with perfect fidelity - they prioritize pacing character arcs, incentivizing characters to express (or challenge) their imperfections, moving the spotlight from PC to PC, and forcing PCs to choose between two things they want. Depending on the game (and the preferred drama ported in with it), that choice might be "choose to save the people you care about vs kill the horror forever" or "choose your character's tragic flaw vs the welfare of an NPC they care about". A decision that would be a ruthlessly smart "got both of the things I wanted" move in an RPG might feel like a copout in a story game, because the point of character drama is that you can't have it both ways.
Another PbtA mechanic that supports "making a TV episode together" type play is the convention that on a partial success (7-9 roll in AW), you get what you want with a complication ("with strings wicked attached"). You know how when you watch a good television drama, they don't just have every scene be the main plot, and they sprinkle in detours and side stories and sometimes link them back to the main plot? Success-with-complication is a reliable way to seed those B-plots, and how they come back to bite the main plot. If you add a ton of entanglements like this in an RPG (say, the Star Wars d20 I'm playing with my friends), this gets hard to keep track of very quickly, especially since most RPGs already produce mini-conflicts and problems like "you rolled a 1 and broke your x, logistics and tactics demand you take a detour to replace it or you will be vulnerable." It works fine in PbtA, though, because the whole game is structured for you to improvise entanglements and follow them up. tl;dr RPGs run on logistics and tactics, "story games" run on dilemmas and entanglements appropriate to the genre of fiction. Different skillsets that cluster in different audiences!
Which brings me to the lack of GM guidance:
A lot of the GM guidelines for PbtA games live in knowledge of the genre the PbtA game is set in. Like
@Henri Barbusse said: if you've read Twilight or watched Buffy, you have a good idea of what problems, dilemmas, and characters make sense for a Monsterhearts game. You draw on those things to GM the game - both to make suitable conflicts, and to make sure the players are making suitable characters. The rules help you pace the way you deliver the conflict, and balance the way PCs pass it around. (I'm not endorsing MH as a game; the "queer puberty" angle and the sexuality mechanics are creepy and not in a fun way. Makes a lot of sense when you realize that designer Avery Alder, power word Joe McDaldno, is a troon.) Here are two other examples:
- Apocalypse World - the post-apocalyptic genre is about scarcity, competition, and how you can't rely on anything to last now that the Golden Age is burned to the ground. Most people understand this intuitively, but if they don't, they can watch Mad Max. If you have a good idea for a ruined landscape and the groups who live in it, the game mechanics and directives will help you create the dilemmas from the first sentence.
- Masks: A New Generation - made to create superhero coming-of-age stories. A lot of superhero media that you might've seen is already a coming-of-age story; the game cites Teen Titans in its inspirations, and a handful of comic book characters in the inspirations for each Playbook. You make up the villains, grown heroes with their grown expectations, and normal people the PCs would be connected to (or port them in from your favorite franchise), and use the rules to make the character drama you'd expect for a superhero story about teens growing up.
This also works with settings that are more on the "detailed" end than the "evocative" end - there's a Bleach-inspired fan supplement for Masks called "The Worst Generation", which details its own factions, places, villains, competitors, and themes. This is vital information for more vague genres like "superpowered battle shounen manga", which cover a broad range of conflicts, power scales, drama, and outcomes unless something nails them down. What better to nail them down than a detailed setting that gives you all that information, and delivers it in a flexible enough format that it can work for an improv-heavy PbtA game?
This brings me to why a LOT of small-time PbtA games fall flat, even when the whole group is seeking a "group fanfiction" type experience: they're not grounded in any meaningful setting or genre. Thus, the game lives and dies on whether the GM can effectively slot in their preferred media or original creation to answer "what kinds of characters, conflicts, and outcomes make sense",
and how effectively they can communicate it to their group. "This game is copyright-friendly Naruto, and your characters are all young adults and chuunin" comes preloaded with a set of goals, problems, side characters, and flavor. "This game is based on shounen media about camaraderie and effort" is a recipe for option paralysis, lack of focus, and "is your guy overpowered, or are you playing JoJo while we're playing beginning-of-journey HxH?"
This is a long explanation that serves to say: it would make business sense for PbtA and adjacent games to simply market towards fandoms (books, TV shows, etc), rather than marketing towards RPG spaces as a whole. If you read the entire spoiler, you understand how both the audience and the skillset suitable to play those games is closer to that of collaborative writing or fanfic than AD&D and wargames.
