"Why can't I rip apart the UI of an app I use every day and rearrange it the way I want?"
Because it is very hard to do something like this, so common people can do it. (you can change every html UI in theory)
I tried to make something like this and basically failed (though in the very long run I might get there eventually). GUI editors are hard to get right and the ones I liked, like Adobe Flex Builder (with Flash UI as a bonus) are gone. But those were also no newb tools. But flash itself was and that was the main reason for its success.
The typical Old School Unix way to do this is to provide all of your application's functionality and business logic through a command line app or at least an API, with the UI being a thin layer on top of the command line. Then anyone can build whatever UI they want on top of it. We've fallen from the light and now the prevailing design is to deeply integrate the business logic with the UI to the point where they are codependent and inseparable.
In some ways web apps act like this: all (most) of the buisness logic and data is on the server.
And you get the data through API calls - so in theory one can build your own UI on top of a known service. There are rare examples of this done succesfully (for HN for example), but usually you won't get very far in a reasonable amount of time, because often it is a mess behind the shiny UI. (And because this is not encouraged behavior by the service provider)
as sibling commented, i look at the Automatic-1111 stable diffusion "webui" as being exactly like you describe, where the "default" service is built in, but the API seems relatively straightforward enough that people have build electron apps and android apps that can talk directly to the python service behind. one of these days i'll actually code a useful mobile UI for my servers. Until then, the reflowing webUI works "ok enough" on mobile.
another one that i'd like to think has an open enough API is mastodon/fediverse, assuming they actually adhere to the activitypub spec, it should (should) be relatively straightforward to write a UI on top of the API. And it seems ok, there's quite a number of mastodon "apps", but less for stuff like misskey or pleroma.
I'm sure there are other contemporaneous examples, but suffice to say that some developers still care about this, today!
Because it is very hard to do something like this, so common people can do it. (you can change every html UI in theory)
I tried to make something like this and basically failed (though in the very long run I might get there eventually). GUI editors are hard to get right and the ones I liked, like Adobe Flex Builder (with Flash UI as a bonus) are gone. But those were also no newb tools. But flash itself was and that was the main reason for its success.