That's not necessarily true, I have a feeling that I can't really back up that ~all infectious respiratory diseases are airborne (i.e. the infectious particles

That's not necessarily true, I have a feeling that I can't really back up that ~all infectious respiratory diseases are airborne (i.e. the infectious particles travel between rooms and linger in common settings), but some combination of infection routes, innate and learned immune responses (actually quite fuzzy when you look at them closely), and human behavior (e.g. "alternate explanations" that epidemiologists come up with like sneezing/coughing) explain why most people don't believe that's the case.