It seems to me that the perception of boringness can come as a result of earnestness being very niche/focused. Your example is a great one! But if anybody who is not a fan or doesn't know Borges, they would probably find a person earnestly passionately about his writing quite boring -- as one can only have the time and mental energy to be truly earnest about so many things.

In contrast, GP's example of popular people are likely those who are good at faking that kind of deep interest in many things, tailored to their present company, so that they attract a few people from many different crowds. Of course, this is based on my own experience only; in certain circles I can feel quite loquacious and come off as extroverted -- usually because of an earnestly shared interest -- but in most cases I think I come across as boring or even introverted!