1. I am sure there are historical reasons for the way things are.
2. Qt is not a platform. XDG/Wayland/Vulkan/PipeWire/D-Bus is the platform. Qt is a library. The platform is (relatively) lean. And the library is more complex. Whether an app is written in Qt or GTK or anything else it should be transparent for the end user. Whether an app is an executable or a website is not transparent, hence different platforms.
3. Rendering in terms of HTML/CSS is certainly valuable, but it can be reduced to a small subset of what it is + libraries.
4. This reduction would be very useful.
5. In this counter-factual reduction, one can just invent a better method for library deliverables, although I suspect a long TTL chached GET from a cannonical source would be much more than sufficient.
> That's a luxury not afforded to browsers.
That's a luxury not afforded to platforms. You cannot remove core Wayland protocols or core Vulkan functions or Linux syscalls.
This generally isn't an issue if you don't bloat them in the first place with things that don't belong in a platform and that belong in a library.