Y...probably due to character sheets having a blank line for "Deity" as though everyone, not just clerics, was expected to pledge their loyalty to one. (I know the 1E goldenrod character sheets made me and my friends think that was what was supposed to happen.)
Yes, I remember that line on the goldenrod sheets. And often enough, we did choose single deities.
But then, it also has spaces for "Spells Known", if I recall correctly - and that didn't imply that fighters should have things in those spaces. And we didn't always put in the player's name.
More to the point, though, is that those were PCs. Back in 1e, NPCs (so, the world lat large) was understood to work under different rules than PCs. So what I write on my sheet doesn't imply that everyone in the world has it.