I don't see how that's true, you can absolutely do the same thing on the desktop? No one is forcing you to use the OS's default widgets there either, you can use Qt, Flutter, or even paint every single pixel yourself which is a lot less feasible on the web.

The main reason the web won is that you can ship SaaS without the user having to install anything locally, and there's no expectation of it being able to work offline, so you can offload processing to servers resulting in fewer bugs, compatibility issues, and DRM problems.