Because, again, it’s kind of hard to argue against what they literally carved into a rock.
Not arguing with you here, cuz I have no dog in this fight, but you’re seemingly ignoring the possibility of the emperor bragging about crowd size the number of slaves workers utilized?
I’m not saying they didn’t have a lot of slaves, just thinking that they might be exaggerating slave count (as a metric of how powerful they were) while using something like this river (something innocuous that they wouldn’t need to brag about) to augment the bodies in use.