My bad the article said seasonal rains, not torrential. We have stone carvings in scandinavia too but I assumed sandstone was much more susceptible to weather. If not the rain then wind and sand alone over 8000 years would wear it out.
Mesa Verde as one example includes such cultural works dating back to 600CE. https://www.nps.gov/meve/index.htm
But how can we know if that was deliberate or a result of natural erosion weeding out the covered carvings from the uncovered?