It's difficult to explain, but in questions, we usually use "so" instead of "very". "Very" is related to the word "verify" and means "truly", "so" is the answer to the question "how" and means "like that/to that degree/begitu". In a question like this, the emphasis is not on being boring, or else we would have asked "why are you boring" without a modifier, but on just how boring that person is, the degree of boredom if you will. We are asking "why are you boring to this degree" as opposed to another, lesser degree, not "why are you truly boring" as opposed to not being boring at all, because then "very" wouldn't be contributing anything to the meaning of the question. "Why are you very boring?" sounds like somebody is making a distinction whereby they consider being a boring person and being a very boring person two distinct states of being that don't exist on the same continuum. It's likely to be used for comical effect to emphasise just how uniquely the boring person is being boring, but even then, it's more of an ironic turn of phrase that people have to keep reinventing rather than something entrenched in the language.