We're presumably speaking about a shared bathroom in your home, not about travel accommodation. It would infringe upon a fundamental need for privacy that people have: having a space that is yours alone, where you are free to do whatever, without being observed by others.

I've seen some (studio) apartments that have a private bathroom that outside the front door, but still inside the building. I would very much dislike this: to me, it feels like it would break my private space, forcing me to allow for the possibility to run into others each time I go to the bathroom.