> barely any plumbing (despite this image of Roman engineering)

Nobody in their right mind would have even wanted plumbing in their home at the time.

Plumbing of the time was not airtight - this was before cheap metal and S-traps. So any drainage would be a highway for noxious odors and gasses right into your home. Bringing in fresh water would only be marginally useful without some sort of drainage.

Outbuildings persisted in the West for a while after modern plumbing because unless you are acclimated to it, the very idea of bringing refuse facilities into the home goes against every natural human instinct.