I think a big part of the reason why on-screen evidence is taken as literal gospel in Star Trek, is that several publications, such as the 'Star Trek Encyclopedia'
and 'Star Trek Chronology'
advocated this strict interpretation of canon.
People, like me, grew up reading those reference sources, and therefore, the idea of a strict visual canon became embedded in the community. Websites like Ex Astris Scientia further reinforce this, by using the same standards outlined.
So blame the Okudas