I think there is simply too much stuff to know about in both coding and design. There is simply not enough not time to become really knowledgable in both. It's

I think there is simply too much stuff to know about in both coding and design. There is simply not enough not time to become really knowledgable in both. It's actually "worse" than that. It's pretty impossible to be expert in one field. I don't know anybody who is expert in, for example, embedded development and game development and back-end web development. I don't know anybody who is expert in, for example, font design and user testing and icon design. Yet developers and designers can be successful by picking subsets of "design" and "development" to be expert in. There is nothing stopping folk being expert in subsets of "design" and "development" - apart from the notion some people seem to have that one skill set interferes with the other.