> The writer presents little numbers on prevalence.

Imagine if you wanted to obtain information about the bathing habits of people today. Where would that information come from? The universe doesn't simply provide it. People have to collect it! The do polling. Polling for bathing behavior wasn't a thing in the medieval period. It isn't like the numbers existed and were lost, they never existed in the first place.

You are thinking about history in completely the wrong way and expecting things that cannot exist.