I’m not sure “brainy” people are that much more likely to think they can make good decisions than any other politician. So why do you feel this would exacerbate the hubris politicians already have?
Have politicians ever been voted en masse for their ability to navigate the domain they will be deployed to work on? It's a combination of popularity and (stated) opinion overlap.
Maybe they are just less experienced in politics. I wonder if their education includes much about the problems they need to solve for society in that country. On the plus side a PhD is good at research and can probably get up to speed damn quick and read a lot. As long as they don't have the hubris!
Many if not most career politicians have education in their domain: law, civics, economics, political science, or similar. On the other hand it’s a common failure mode of STEM people in particular to assume their expertise can be directly applied to more "human" fields and indeed to dismiss those fields as unscientific hand-waving.