I don’t think that’s true. I think @tbernard was on the right track.
Left to their own devices many developers prefer working on performance, plumbing, etc
Development and UX aren’t mutually exclusive. My whole career I’ve worked on full stack web apps with Java backends. The enterprise companies I worked for weren’t paying for designers so we developers did all the UI/UX work (though enterprise apps don’t have the best UI either) Open source generally doesn’t have good design because the devs choose not to prioritize it.