An architecture concept just needs a page or two of docs and diagrams to understand, and the code is right there in the app. You don't need a framework to implement relevant design patterns.
Those apps can also last longer, because there's less pressure to rewrite in React/Svelte/Solid/etc when the existing code isn't dependent on some obsolete framework - Ember/Angular 1/etc.
i.e. a framework.
> Then because they're complicated they're difficult to learn.
It's easier to learn a publicly available framework, as there is lots of help on Stackoverflow, and usually the popular frameworks have good, or just existing, documentation.
> existing code isn't dependent on some obsolete framework - Ember/Angular 1/etc.
The "app architecture" can also become obsolete. But now instead of a migration path, you're stuck with the person who made the "app architecture" most likely having left the company.
I've adoped lots of code, some good and some terrible. Framework-based code is only easier to understand if I already know the framework and sometimes not even then if the original developers mostly had to fight their framework.
The "app architecture" does not become obsolete because nothing has come along to replace it and it was already simple enough that Stack Overflow was never a necessary part of development.
K.I.S.S. Not just glam metal.
Furthermore if you're trying to do anything complex and you don't use a framework, you just end up creating another framework.
But this time there are no updates or help on Stack overflow for the person who inherits the code.