I don't know about "coming with their guitars", maybe they did. But, I never saw any strap "come with" a guitar back then, except the "folkie" guitars that sometimes came with a string. Sometimes a store owner might "throw in" a cheap basic strap to sweeten the deal.
AFAIK, those "basic straps" were the only thing available back then except, of course, for C&W artists with their names on theirs, and ... ...
.
Yeah,
that is precisely
why they are still available today.
I never saw, or heard of, anything else, non-custom, until Hendrix came along rocking these things. Then it got
interesting!