I think it often boils down to everyone having an opinion about design/UX. When fixing a software bug in a PR there is often a fairly clear subset of "correct" solutions, but when it comes to design/ux the set is generally bigger and everyone (even non-designers) will have an opinion on how it should look.
Hm, maybe, bikeshedding is a killer in these things... I haven't even seen people attempt it, though. Maybe they all got scared away by us developers years ago?
