I think one of the big reasons for over-classification is that people often only learn things in a classified context. So if you only ever see F=MA in a classified document you'll kind of assume that it's a classified formula.

Also when classifying things you do have to portion mark but you don't need to justify any of the portion markings so you don't have to actually know if/why something is classified to mark it and that causes markings to perpetuate.