Because a lot of people who are "intelligent" aren't really all that intelligent, its just that there brain works really well in a certain realm. I believe a true intelligent person can also navigate the social realm if and when they want to. Doesn't mean they are a social butterfly, but can have decent social skills. True intelligence can span beyond math or physics or chemistry and into every aspect of life including being social.
Take what I say with a grain of salt, because that's all it is compared to the ocean of complexity when it comes to actions and real life.