I was discussing this issue with a friend the other day. I think the real 'why' is there is no motivator to become good at both design and development. Both skills take people who are passionate about what they do, usually to the point where they hone their skills in their free time. I actually think most (good) devs and designers have slight inclinations towards each others work, but it is a serious effort to go from "that does look cool/useful" to professional proficiency. The few people who are currently 'designineers' are mostly likely completely passionate about both (which is their motivator). However for the rest why put in the effort? My understanding is that the pay isn't much different (in fact this sounds a lot like, "wouldn't it be great if I could get 2 for the price of 1"). Easier to find a job? Perhaps, but right now I think good designers and developers are having no trouble finding work. Additionally your skill set is now the target of tons of people looking to get something for nothing, or have one person doing 10 jobs