Because, at least in 1 edition, I could do exactly that without having to do any house ruling or any futzing about with the mechanics. I could run any module, any adventure, and not have to worry about it.
It's been a long, long, time since I played 1e but I'm extremely skeptical of this comment. Most of the adventures I remember assumed a level of magic and especially a a level of healing that would be impossible without a mage AND a cleric in the party.
While I'm sure adventures existed that didn't require such, the "mainstream" adventures (such as T1-4 or the giants series) assumed you had mages and clerics in your party and success without them was extremely suspect without heavy houseruling.
I get what you're saying, but baseline D&D has always assumed a high level of magic, the big difference is that earlier editions concentrated the magic in fewer characters.
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