I believe this happens because writing high quality UIs which is consistent, is well studied (as in having studied how users react to it in order to improve the

I believe this happens because writing high quality UIs which is consistent, is well studied (as in having studied how users react to it in order to improve the worst parts), and is available for multiple platforms, is a HUGE amount of work that would only go forward if people got paid to do it. So I'd wager that lots of OSS have bad UI because no commercial entity considered that the market is big enough to justify spending resources on it. This is, an external company pouring money at hiring OSS devs to work on the UI, or maybe even the project creators themselves founding a one-person business to sell services or products that require such a good UI. An example: if Mumble had a huge user base, maybe there would possibly be some company selling high-quality, easy to use desktop applications for it. But reaching that goal is very hard, and most OSS projects never get even a fraction of the traction that would be needed for such commercial efforts.