Well, it's obvious they really wanted to though. This isn't like marking on a wall with a Sharpie. They had to work at it to make it happen. The commitment alone is impressive even if it was just a "Grogg was here!"
It actually makes me wonder if this technique would produce any results in the notorious Erdstalls in Germany. I suspect not, but no one has any better ideas, so it might be worth checking.
It seems quite strange that they could be that prolific while requiring that much manual labor to construct yet there's no written reference to any of them.
> "No matter what you do, when we find something weird in ancient history we will decide it must've been part of a fertility rite."