Osee
Does this have something to do with that people like putting "longer" words at the end to draw attention. For example, when starting a speech, people say "Ladies and Gentlemen," where the world gentlemen is longer than the word ladies. While in "Dear Sir or Madam", Madam is longer than Sir. Just for fun.
I think the reason for writing "Dear Sir/Madam" is that in the past, when we didn't know whether the Director, Manager, etc was a man or a woman, we wrote "Dear Sir'. Later it was felt that it was unfair to write 'Dear Sir' because the person in authority might be a woman. In older grammar books, the advice is to write 'Dear Sir'.
Formerly, when we wrote 'he', it was understood that opposite sex was included. But nowadays we write he/she, he or she, or s/he.
A woman uses Ms if she doesn't want us to know whether she is married. Formerly, only 'mankind' was found in dictionaries. Nowadays, 'humankind' is also found.
If I'm not wrong, the above came about because of Women's Liberation.
This is my opinion. Other members may disagree.