In a functioning software company, the managers recognize this skill difference and simply order the developers to implement what the designers suggest. They grumpily do. (It’s obviously not always so stereotypical. I’m exaggerating to make a point.)
In a typical OSS project, no one can give orders. The programmers ignore any good design suggestions because they don’t recognize those, and implement their own crappy ideas instead. Sometimes while arrogantly dunking on the designer who tried to make a contribution. Consequently, UX quality is abysmal, and anyone who could fix that is driven away. I don’t see a solution to this.