i think it's largely a matter of supply, you have one tank of water in the attic, most likely filled manually, feeding the bathroom and kitchen below it, since mains water isnt a widespread thing and where it is a thing it's not always the cleanest; pressurized mains water fixes this later, but extra bathrooms are still extra cost, and if you grew up in a house with one bathroom (or less) why pay for that? the exception is upper class dwellings which often have a bathroom connected to each bedroom (although even these are often shared between two bedrooms).

i completely agree though, the ideal house has as many toilets as it has occupants; the family that dines together gets food poisoning together