I thought I remembered an early D&D book explaining why they went with "he", but I guess it wasn't the 1e DMG or PHB doing it. Was kind of surprised that the DMG has "he or she" 314 times. PhB has it 263.You know how most gaming books, up until recently, used the male pronouns for everything, largerly because the default assumption was that the readers were guys. But then people realized that hey, women game too, and started including female or actually gender-neutral pronouns as well.
Does more need to be shown to claim that use of "he" meant it was because the authors didn't think women played? When did "he" stop being taken as the third person singular of unspecified gender in the style guides and English books so that "he or she" became a thing? (Or am I misremembering how relatively recent that was?).
I'm still rooting for singular they to be the default and am glad it's hit more of the style guides.