I am not a researcher, but I have a simple evolutionary theory that soap was invented in the last few thousand years and became a mass-market product after the beginning of industrialization.
If we survived and evolved without the use of something in the last few million years, then why is that thing needed?
Because we didn't. A lot of people died, actually. From germs. Before we knew about Germ Theory.
I see this same type of stuff when people talk about inductions or cesarean sections. "Well humans didn't need that before, so why do we need it now?" No actually... humans did need that before. Half of all infants died. Humans are unbelievably shit at giving birth.
Turns out, humans are bad at a lot of things. We die A LOT less now. Like... so much less that we can't even conceptualize how much less we're dying so then we start questioning if we need soap.
And, as a fun aside, the reason humans are so shit at giving birth is because of evolution. You ever wonder why seemingly ever other mammal is able to give birth and it's super chill but we just roll over and die in shocking numbers? Yeah, turns out evolving to be one two legs has disastrous consequences.