Because billionaires are an elite, not an upper class
These great houses were built to communicate power and rule, and also order, tradition, and rootedness
Their magnificence showed that the ruling class, which led well and governed well at every level from the vestry councils to the highest reaches of the empire, was willing to invest for the long term and cared about cultivating itself and that which surrounded it
We instead gave an economic elite that largely just cares about money, and has handed off the levers of rule to a cold and uncaring, and generally incompetent, bureaucracy
Why don't modern day billionaires live like this?