Desktop crossplatform development is a weird beast. Yeah you get to share a lot of code between platforms and it's easy to get 80% there, but getting a polished UI is a lot of work.

In some cases such as using JUCE for making audio plugins it's even desired to not have a native UI since users expect each plugin to have its own UI. But unless you are huge, like say Adobe, having a nice and polished cross platform UI for application development is simply not feasible.

After years I've come to the conclusion that the proper way of supporting multiple platforms is having native UIs and sharing application logic in any way you can between platforms. I think that's the idea behind Xamarin.