The real answer is that the techie nerds willing to learn git and contribute to open source projects are likely to be hobbyist programmers cutting their teeth on bugfixes/minor feature enhancements and not professional programmer-designers with an eye for UI and the ability to make it/talk with those who can. Also in open source projects its expected that the contributor be able to pull their own weight with getting shit done so you need to both know how to write your own code and learn how to work with specific UI formspecs. Delegating to other people is frowned upon because its all free voulenteer work so whatever you delegate ends up eating up someone elses free time and energy fixing up your pr.