We get caught up with English being so removed from the underlying symbology, but the older you get in terms of language development the more it becomes a sort of "symbol that looks like ripples = water" sort of language, and the number system that goes along with that is often some form of tally marking that just happens to be in some other shape. A lot of east asian languages are still like this, basically stylized pictographs.

And once it gets invented it snowballs as it is shared. How old were we when we learned to tally mark? 4-5 years old in the playground sandbox maybe? I remember thinking how great it was you slashed for 5 to just count blocks of 5 tallys. But its one thing to learn it from others and understand it. Another thing to come up with the system entirely. No one in the playground was probably clever enough to do that yet. We could learn but few could probably be the doers.