Desktop Slack runs on Electron, no? Which essentially lets web developers develop a "native-ish" cross-platform app, without learning a whole new platform.

There is a long, long history of attempts to make good, cross-platform, native apps. In the late 1990s there was a company called Visix that had such a library that worked on Windows and Unix. There have been Windows implementations of Motif/XWindows. There was of course Java/JWT. These days there is Qt. None of these seem to have really caught fire.

edit: I wrote Mac but I meant Unix