Most guys are afraid of a smart and successful woman.
Why?
There are many women that derive their self-worth from the way they look, always insecure over their weight, clothes, et cetera.
Many men have the same thing, but with status. They derive self-worth from having a good job, getting respect, being seen as a leader, that sort of thing (look at all the guys above immediately going "what!! I'm not afraid!!").
Of course there's nothing wrong with either of those, it's just the way people are. Biology. Comes from how we choose mates.
So in the fictional ideal relationship where you both increase each other's self worth to the max, the man makes the woman feel beautiful and sexy, and the woman makes the man feel smart and powerful.
And although it is very cool to see a smart and successful woman, I love them, my wife is one, it does mean one more point in a relationship that's slightly less smooth than it could be. Because often both are trying to lead, and it can feel as a direct hit to a man's self-worth if he happens to "lose a fight" in a way that it doesn't for women.
Of course, there's ample things that make up for it :-)
One reason that males have a difficulty with intelligent successful ladies is our male ego has a problem because we want control and we realize that we have lost some control when we are around people more intelligent than we are.