To even moderately experienced developers, a UI is a tool we use to interact with an application.

To everyone else, the UI is the application.

We developers need to develop an novice-level understanding of many things we touch but don't specialize in... Perhaps the domain we're developing software for, or networking, or database work. We also tend to get the classic novice overconfidence in those topics because we've never needed to know as much as the experts and therefore have never encountered problems we weren't qualified to solve... or so we think. UI design is one of those things.

I'm an art school educated UI designer, have done significant work in serious graphic design, and worked for a decade as a full-time back-end web developer for a reputable organization. The number of developers that have tried to explain UI design to me, even knowing my background, is astonishing. I'm male, but it looks exactly like mansplaining by my estimation.

UI design is a profession totally separate from software development and graphic design because making a good UI is a totally different process than either of those things– it's more akin to industrial design. Most developers don't understand its importance, and vastly overestimate their ability to create and evaluate interfaces themselves. I contribute code to open source projects all the time... but trying to contribute design work is a miserable experience. This is the reason independent open source alternatives will remain alternatives and never become standards.

I take supporting evidence from IDE market shares. All of the top results are either commercial software or mostly controlled by companies that have staff UI designers. The one org that controls a top contender that didn't seem to employ a dedicated UI designer is Eclipse, which is famous for its usability... Just not in a good way.