It's not just how the "science" is conducted, or limited to a fixed number of sides. Everyone doesn't quite know what anyone else experiences. We all just throw around symbols, hoping someone gets what we mean by what we say, and assuming that we know what others mean by what they say. The meat of what we know and experience never gets transmitted faithfully to anyone.
To be certain, many people do have conditions that, say, I will never have. But that doesn't make me "normal" or those people "abnormal". The definition of a disorder by showing harm to living one's life is a good start, but fraught with the complexity of analyzing things in an implicit social context. If it seems that someone has a problem, I'll consider it a problem, not only if it seems sufficiently and officially abnormal.