Most GUI suffers from the NIH sydrome. Instead of using system components (and systems values and colors), every designer are pushing for adhoc widgets with fixed values.
Nobody can decide on standards because everyone is greedy. Microsoft has, like, a dozen GUI platforms and they're all Windows-only. Apple doesn't even use an off the shelf rendering API. And android is... Well, android.
Sure, I would like to use the OS provided controls. And then maybe after that we can all hold hands and sing Kumbaya.
The only reason the original post think that "used to be a solved problem" was because we used to have a standard for guis: win32. But obviously people wanted more powerful/flexible UIs and more importantly, cross platform guis so it eventually died off.
A lot of design innovation came because the web is extremely flexible while at the same time being extremely non-standardized.
I think it's because UI design is very much something that can't really be "solved" in the traditional sense, so a lot of us use our own opinions when we inevitably find a corner case that isn't working exactly the way we want.
Most standard toolkits provide escape hatches to have your custom components. I think most designers wants their view imposed to the users instead of following the platform constraints. Like instead of the standard play/pause button every player have, they want their own custom ones. Instead of using the standard tree widget, they want to create another that is the same, but behaves a bit differently. And then they say GTK is not good, let's go with HTML in Electron.
Greed is maybe not the right word, but maybe it's more like ideological greed. Like everyone wants things their way 100%. They won't settle for 99%!
Like okay, with Apple and vulkan and metal, is vulkan the best API? No. Is it the most performant? No.
But it's pretty fucking good. Its 95% of the way there. But for greedy apple, that's not good enough. So they say "fuck you, heres metal, good luck developers!"
And now people only make Mac apps if there's a gun to their head and they really need that tiny slice of market share.
Its the same thing with Qt versus bespoke UIs in ImGUI or whatever. Qt is really, really good. Its not perfect.
But we're really gonna burn it all down and start over for like... a 1% improvement in code architecture or something? Really? We're chopping off both our legs so that our pinky finger hurts slightly less?
Sigh. Okay. Such is the way of developers, I guess.
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Also, don't forget that if you want to submit the values with something else than a GET wirh URL params or a POST with formdata mimetype for the body, you'll need JS anyway.
You can use it as backing for some JS component though (visually hidden), this is often done to have components work accessibly and with maximum interoperability. And to get functionality and tests right without implementing complex ARIA standards perfectly and running into constant issues with vanilla form submits (see above) or focus states and a11y.