And, yes, I know there’s some thing people did long ago to set the speed limit at some percentile of what people were already traveling at, but that’s obviously a stupid way to do it and we did it anyway. So why?

I'm going to disagree about that being a stupid way to do it, for two reasons.

First, there was a time traffic engineering was not a well-developed field. There was no expert who could combine science and best practices to get a good answer about how fast it's safe to drive on a particular road because humanity had not yet learned that skill. Measuring what most drivers do, then reducing it if people crash a lot in a particular place is far from the worst approach.

Second, it's bad to make laws that most people will only obey with heavy enforcement. It's bad for a society to make lawbreakers out of most of its members, and to create pretexts for police to stop people without other evidence of wrongdoing. If there's a reason to make most people drive a particular road much slower than they actually do, any reduction in speed limit should also be accompanied by changes to the road design (traffic calming).