[c-smile]

> Why isn't there a good, open, cross-platform UI library already, that compiles to the native UI of each OS?

It is either "good native" or cross-platform. But not both in reality.

In principle can do something very basic using stock platform widgets - some application that uses only basic widgets: buttons and plain text textareas - all others are too different on different platforms.

If your app is slightly more than that you will have problems even with basic stuff. Consider this native UI example (Notepad++) : https://i.kinja-img.com/gawker-media/image/upload/c_lfill,w_... and something like Sublime Text

Notepad++ is a disaster even native platform API. Sublime Text uses custom non-native renderer - consistent UI styling.