My personal belief is that for the author of AD&D 1e, the game and the setting were inextricably linked. That’s why there’s so much setting specific information in the DMG, and built into the classes. The author was just of a high level enough to know how to genericize it for his readers, so that they could port and customize it to their personal games

In fact that paragraph just gave me an epiphany. The whole reason I switched from playing 1e RAW to 2e was... I didn’t like Gygax’s setting specific stuff in the PHB and DMG and MMs controlling my game