Because the skill sets aren't the same. Research in of itself is a different skill set. Getting a PhD is more about learning how to take abstract and vague ideas and turning them into a reality. Making highly readable, robust, and secure code is a different skill set. Sure, you can have people that do both, but at that point you're often asking a painter to do sculpting. Skills do translate, but not well. They probably won't be interested in it either, so you're not using your workers efficiently either. Give the canvases to the people who love painting and give the marble to those that love sculpting. You'll probably get better art in the end.