This doesn't work, though, and we can use Qt and wxwidgets as the reasons why.

Qt has tried a lot of different ways to render GUIs. Traditional qwidget applications use, depending on the widget object, either native platform specific controls that you try to uniformly mutate despite the stylistic differences, psuedo-themes Qt writes for non-standard elements or things that aren't universal and thus you need a facade for where they try to impersonate the native toolkit (this is why so many Qt programs on Windows "feel wrong"), and user-styled controls that don't even attempt to be native but you can mix those freely with the base widgets that emulate the native look and feel. This is how VLC can have its volume slider look the same everywhere.

Then in Qt4 and much more comprehensively in Qt5 they moved to QML, which completely drops the idea of trying to use native window systems and renders the entire UI with 3d acceleration. They also, at first, made no attempt to even provide common toolkit elements - you got basic drawing primitives and the ability to query get a color pallette that you are told by Qt should be the "native" colors.

Turns out not a lot of people took off with that offering so Qt developed QtQuickControls1 - which was a giant mess of depending on qwidgets to map native widget styling into the OpenGL scenegraph they had. It actually did somewhat work, but only insofar as you got buttons that looked native - go try resizing these Controls 1 windows and you discover a lot of incongruency, especially on less practiced platforms like Android, especially where faux-native widgets and GL components mixed.

QtQuickControls2 I feel is an accurate representation of what all GUI development has to be - if you want native look and feel you do need to do some manual management to get it, making your own themes and having logic to change the design to accomodate the target platform. But it also is meant to make good looking software by using the style guides of platforms rather than the lower level actual widgets - the original release has Material and Universal from Google and Microsoft respectively. They don't try to look native, but they try to preserve the stylistic guidelines of the host platforms. They later added a few other options - a "desktop-like" theme, one that you can easily restyle with generic images, and a default that is super plain.

Wxwidgets presents the other extreme. It tried super hard to never implement a widget itself - it was always meant to be a facade over native toolkits. In other to accomplish that they had to sacrifice ergonomics and introduce means to distinguish the platform native widgets that don't map cleanly across platforms. You never get a perfectly universal cross platform UI this way, but you also don't benefit from native toolkit workflows because you are trying to abstract something that isn't a 1 to 1 mapping.

There is one more aspect to all this - native toolkits generally blow chunks. And that precludes how Windows has at least three actively maintained "first class" GUI toolkits now. All first party toolkits today are derived either from bitmap or raster graphics, mangled over years or decades to support new paradigms that they weren't meant to, and to this day have errant behavior abound and little to no consistency due to their backwards commitments.

That would not have been a problem if we were all still operating on computers without GPUs with displays from 1995, but the evolution of hardware keeps pushing on our desktops to take advantage of the resources available - in particular, GPU acceleration. 2d acceleration is completely dead. If the world made sense and legacy could be eschewed for reason every desktop would be from the ground up built on 3d acceleration. I haven't checked in a while but I believe it wasn't until Windows 8 that Microsoft dropped the non-accelerated bitmap Windows desktop for 2d acceleration. KDE on Linux went through a huge churning to migrate their desktop to 3d acceleration and that still only applies to their newer tech - anything QML based - while the bulk of KDE applications are stuck being unaccelerated qwidgets programs. Gnome is in the same boat, and never made the leap to a 3d UI paradigm across the board that they should have made when GTK3 broke everything.

Because mixing these two worlds is just an exercise in suffering, but every UI design on every platform either has to wage the war or just succumb to using the broken old bitmap style with hacks to try to make DPI scaling work.

Not all the blame should fall on UI designers though - this is a structural problem. Before Vulkan we had no seriously universal and working acceleration API. You could not rely on OpenGL a decade ago much less now - just look at Godot, whom released their 3.0 with OpenGL ES 3 and is now forced to adopt an ES2 renderer since so many ES3 implementations on both desktops and mobile are broken. There was simply no universal API to ground the foundations of a "next gen" 3d toolkit in, which is why Qt supports a myriad of backends to QML including an unaccelerated one they were forced to implement because so many edge cases broke the renderer.

Call it the 14 + 1 standards problem, but whoever writes a declarative toolkit built on Vulkan that can be like QML except without all the attached baggage (theres a reason the Rust QML crate has been dead for over a year) would be my choice any day. Its just a hard engineering problem, not just because Vulkan is hard, but because providing simplicity to start without constraining possibility and expandability is an even harder design problem.
Maybe the answer will come from game engines? The overlap gets more and more blatant every day - I've been having a lot of fun in Godot and I ask myself why not just do general GUI development in here. Its accelerated, cross platform, and the scripting is a joy. It just lacks API coverage for comprehensive support of what apps expect since its only meant to make games.