There’s also two main plus one lesser issue that are less commonly discussed:
Lack of manpower. FOSS devs often doing it as a side project on top of some other and/or a full-time job, so that even lowers one’s ability of concentrate on stuff like the UI, when you’re already working hard on fixing bugs, looking up things (which is getting harder and harder thanks to AI slop - I once managed to destroy a Linux on my Raspberry Pi while trying to adjust the path variables).
Getting comfortable with the uncomfortable parts of your application. There are many times I haven’t noticed a a very uncomfortable part of my GUI after months of use, then I had to refactor things, which obviously took time away from other things. This also affects the users already in the userbase.
Elitism is also a factor. A lot of people like the feeling of being part of a special group, and for them, the steep learning curve is a feature, not a bug. I’ve seen Blender users being angry at the devs for “spoonfeeding” the normies, and letting in all kinds of people. Also just look at OP’s image.