Only Qt never had the funding or resources or will to get native UI right.
Their non-native widgets are always off, in a way that even Electron, is not (because it being web-based means it at least follows familiar conventions for all web users, where a mimicked native GUI enters the uncanny valley).
There are also custom native but not platform-native (meaning close to the metal, no web-based) GUIs like Adobes, Fruity Loops, and others, that look good on all platforms and users have no issues with them.
That's just because it describes its own embedded rendering system.
SwiftUI's syntax feels closer to a markup language than code, though, so it might be more appealing to web frontend devs.
No. Chrome is the only platform that is cross platform by default. But IE, eg, has definitely had behavior differences in the past.
On the other hand, Qt is cross language, but not necessarily cross platform (it excludes iPhone and Android, say)
Qt supports Android and iPhone.
The approach has been tried, and succeeded to some degree. But the web is the only app where cross-platform is the default, and it's truly hard to avoid it. Even if the widgets all look different. Even if nothing looks native (people got used to it anyway).
And to be clear I want the status quo to improve in this regard, especially in terms of accessibility (nobody should have to reimplement eg their own dropdown especially if it breaks eg. keyboard shortcuts, mobile UIs and so on). But I can't see anything from today that isn't either a web technology, or web-compatible (and web-accessible, not the Qt webgl hack that's on the frontpage right now) making it in the long term.