Probably because people mostly thought/talked about these images as aesthetic expression (or at least as signs of symbolic/notional thinking), not as something practical.

It is a well known fact that only $current_year humans are intelligent. All previous generations are violent and stupid. They are too busy clubbing themselves to even bother with the concept of time passing.

Don’t forget that if you see something in the archeological record that you can’t comprehend, there is always a reasonable explanation: it’s ritualistic woo-woo.

(There should be a “Razor” for this)