The difference is that you can see it. If you have two highly skilled contributors with very different programming styles, they can still collaborate within the same codebase given a goal and the end-result is the same for your users. When a code contribution is marginally better or worse in its approach than another the difference is negligible to the project as a whole. By contrast, users notice when a single color is different for design elements. And they complain extremely loudly about it.

On the contributor side:

- Visual style guides require much more time and skill to create (relative to style guides for code) and still generally fail to achieve anywhere near the aesthetic unity of a single great designer with total control.

- If you commoditize your design elements to the point where it is easy to contribute them, then it is no more difficult to do it yourself to begin with.

- Delegation of design work still requires that you funnel it somewhere to be judged on qualitative measures, rather than the pass/fail nature of code

Generally the best you can do is collaborate very, very tightly and then funnel it through a single person in the end anyway a la Python's BDFL in a very trial and error fashion.