Those houses are designed for entertaining and large parties. Any event hall that you go to will have at minimum 1 bathroom for each gender with multiple stalls

Those houses are designed for entertaining and large parties. Any event hall that you go to will have at minimum 1 bathroom for each gender with multiple stalls, 2-6. This is still a home, so they choose not to have stalls in bathroom setting, hence 6-10 "extra" bathrooms. Additionally in the very large houses the bed/bath ration doesn't capture the setting of the bathrooms. There is a house not to far from me that has a connected indoor pool and that has 3 bathrooms for changing/shower. (edited)