UI is the perfect storm of annoying mess for the type of coder that lives solving difficult problems. It's tedious work, takes a lot of effort and iteration but isn't truly challenging, documentation is usually spotty, etc. it tends to feel like a chore if you're not the type who cares deeply about user presentation. Plus, almost every social and mobile game needs lots of it. The supply of people who enjoy the work does not match the demand for the amount of UI work on a typical game project.
