My head canon is that creatures such as ghosts, demons, djinns, ... enter our mortal realm willingly from time to time and sometimes form a connection with a person, who they then teach how to summon them in times of need. This knowledge is then passed down.

So effectively otherworldly creatures are tourists who gave a local their number and now they get bothered by their greatgreatgreatgreatgrandkids.

The Book of Enoch says that fallen angels named Uzza, Azza and Azael taught humans originally.

I like the idea that they're not just symbols, but shapes. Get anything to be shaped like a rune, and it'll touch magic. So two rocks leaned against each other just right might create a trickle of water, or a tree that grows a twisted enough web of branches could, by chance, summon a flame. Then, like with all natural phenomenon, people figured it out! It fits well with the trope that wizards are arcane researchers and scientists, you find in settings like D&D's

It was just probably just some poor guy with magical thinking OCD. "If I don't repeatedly draw symbols in the dirt, my whole family will die."