A hot bath being wasteful because you have to heat water and need a sewer is ridiculous.

Solar water heaters are a thing and grey water reuse is also a thing.

You don’t even need electricity to have a private bath.

In addition, prior to the modern era regular bathing was often a privilege of the elite, not to mention the hygiene concerns of a giant bowl of people soup.

Bathhouses still exist in the form of steam rooms and the like, and you could make an argument about the inefficiency of personal hot tubs, but those are more specific examples of “bathing” as a recreational activity and not a hygienic one.