Because back when it was labeled as an allergen. Doctors weren't seen as often back when so it wasn't common knowledge as it is today. It has just stuck to it. It's now just common knowledge. When you go into see your doctor if you tell them you're allergic to dust just watch their face and you'll get an eye roll or a subtle sigh. That's our face telling you that we were asking you a serious question and expecting a serious answer and not asking you if you were allergic to something everyone is allergic to. That's also us wanting to slap you or already have slapped you in our minds for giving us a stupid answer.
Sometimes when I ask people their allergies I say "Are you allergic to anything that isn't dust or mold." Or "Do you have any allergies, and if you say dust that's not an allergy." You aren't prescribed Clariton for an DUST allergy! Think about it.... :roll: ( Well now that's OTC but it wasn't years back. )