Isn't it the exact opposite? Every single house in the modern world has running water—it wouldn't be code-compliant, in any functioning country, to not have that. That was a high-status luxury in Rome. (It was even a largesse of the Emperor to be gifted[0] the right to have a private plumbing connection to an aqueduct—something considered highly desirable in that world).

The fact people today build inexpensive plastic Thermae as a novelty object, reflects how thoroughly we've solved all the *actually hard* problems of water infrastructure. The formerly expensive parts are now unimaginably cheap, so, we're exploring new places to cut costs that we previously wouldn't think of.

(It's akin to how computer keyboards are now 10x cheaper and junkier than they were in the 1960's–1980's (?), because, the other problems having been solved, that became a new focus of economization. No one would think twice about paying (the modern equivalent of) $100 for a well-engineered mechanical keyboard, in an era when the corresponding PC went for $5,000. The expensive object reflects an economic difficulty elsewhere; and the expensive Roman stonework baths perhaps reflected the costliness of water in general).

[0] https://en.wikipedia.org/wiki/Roman_aqueduct#Distribution