1) Visibility. Code isn't visible to the end-user, so it doesn't really matter how a programmer gets from point A to point B as long as it's bug free and efficient. UI/UX, on the other hand are exactly how your users define you, so it's not something you want to potentially leave to the programmer that just installed Gimp and learned how to use the shape tool.
2) Testing. Good UI and UX need to go through several rounds of highly controlled testing. The "controlled" part is the factor here--a group of guys in the basement isn't going to cut it, and there's no way to verify with open source communities that testing was done correctly.
3) Consistency. Design teams are able to churn out more consistent work once they've found their 'style'; multiple disparate designers are more likely to break that style.
4) Preventing design by committee. Pull request discussions for design changes could turn into disastrous free-for-alls that end up turning Firefox's chrome neon blue and each button red because the majority of random devs think it's 'cool'. Design is more successful as a dictatorship than a democracy.
2) Testing. Good UI and UX need to go through several rounds of highly controlled testing. The "controlled" part is the factor here--a group of guys in the basement isn't going to cut it, and there's no way to verify with open source communities that testing was done correctly.
3) Consistency. Design teams are able to churn out more consistent work once they've found their 'style'; multiple disparate designers are more likely to break that style.
4) Preventing design by committee. Pull request discussions for design changes could turn into disastrous free-for-alls that end up turning Firefox's chrome neon blue and each button red because the majority of random devs think it's 'cool'. Design is more successful as a dictatorship than a democracy.