I wonder if the reason could be that for smart people it is difficult to find many good friends. So the actual choice for most of them is between having only a few great friends (which is better), or having many friends that suck (which is worse). But maybe given a chance, having many great friends could be even better.

By difficulty to find many good friends I mean that for people with very high intelligence the set of their peers is already small enough, and then within this set they need to find people with similar values, hobbies, personality, etc. Even admitting this problem is a huge taboo (essentially you are telling 99% of your social environment "I don't consider you a good friend material"), so many people probably don't have good strategies for solving it.