I'm not saying it's a matter of desire. It's a matter of time. A full-time developer has to feed their family, so they have to put most of their time into the stuff that makes them money. That means that their passion project is just naturally going to get less time as a function of the number of hours left in the day and the amount of energy for coding that the developer in question has.

Further, ux design is a less "atomic" process; small amounts of time working on ux is going to have less impact than small amounts of time in coding. A programmer could conceivably fix a bug or make a minor improvement or feature request in ten minutes, and a Wikipedia editor could spend ten minutes improving the grammar and punctuation of an entire article; but the ux process requires mockups, iteration, asset creation, and coding for every change—and even if that can be done in ten minutes, the rest of the ui will look completely different, meaning that the overall ux will be worse than before, despite that one thing looking better.

What can we do to change it? Companies that rely on FOSS should donate to projects so that the people who work on them can afford to do so at least part-time, or empower their own employees to contribute to FOSS on company time. Those are really the only two options, barring some sort of UBI or public grant for open source software.