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