Because most smart people are not generalists. My first boss was really smart and managed to found a university institute in computer science. The 3 other professors he hired were, ahem, strange choices. We 28 year old assistents could only shake our heads. After fighting a couple of years with his own hires the founder left in frustration to found another institution.

One of my colleagues was only 25, really smart in his field and became a professor less than 10 years later. But he was incredibly naive in everyday chores. Buying groceries or filing taxes resulted in major screw-ups regularly