I find it really frustrating that a ttrpg game needs to follow the supplement treadmill model in order to maintain a player base. The only reason to play ttrpg over video games is the freedom to use your imagination, not being told what to do.
It's just the way it is dude.
RPG's are all about their player network effect. They need to constantly bring in casual players to filter through to the hobbyist GM's that will keep the gaming network going. The supplement treadmill serves to constantly keep the game visible, and keep current and potential new players engaged by new offerings.
It is all about continually engaging people's interest. And for various reasons if a game line isn't actively supported, the majority of gamers just aren't interested.
Even D&D has to edition treadmill every 10-15 years to reinvigorate its ranks...
There's so much potential and diversity in the various genres and it's just not being explored because... gamers aren't adventurous, I guess? The ttrpg rules are just so prohibitively costly to learn that the only option is make 5e-compatible products even if the rules don't convincingly support the intended atmosphere?
Your guess is correct. Because...
In General:
Most gamers
do not go out of their way for anything; they talk about what they talk about because it got shoved into their face by virtue of being the mainstream thing, and anything that isn't shoved in their face might as well not exist.
Most gamers
don't like to 'work' for their entertainment; they like to just sit down and play. The game that gives them the best odds of being able to do that is the one
with the dominant network effect. Once they find a game that allows them to just sit down and play, that satisfaction is met. End. Of. Search.
Because most gamers don't like to 'work' for their entertainment;
they aren't curious, so they don't seek things out. Which means that if you don't toil to thrust what you have to offer into their faces, then they will not even consider the possibility that an alternative even exists.
You can see this effect in many threads on this forum of gamers and GM's refusing to look beyond D&D/5e/d20 based games because they do not have the time, or want: "to learn new rules"...
It goes without saying that many people here, myself included, are exceptions to the general rule, and are outliers in the overall hobby.
just want to point out that PbtA really isn't a system, but a design approach. Avatar Legends plays very differently from a lot of other PbtA games, including ones by Magpie. Bluebeard's Bride, for example, isn't just different because of the setting and premise, but mechanically really, really different.
Everything listed is a one stop shop complete RPG.
None of them are following the old Gurps/Hero model of 'super-system tome' + genre/setting supplement. Those are all very much following the d20/OGL, or 3pp license model: "I think my system can be used for various genre's - here's my OGL, or hit me up to be a 3pp."
Oftentimes those games are modding the base system in ways the Gurps/Hero IP holders would consider anathema if done to their systems.
Yes, Cortex and Cypher have their pro forma tomes (The way other systems have an OGL or SRD). But those systems real bread and butter is offering complete, genre specific, RPG's.
No one is following in the footsteps of the 'Universal RPG' Gurps/Hero model, and with good reason.