So what you're saying is that we could already have plumbing robots if more people elected to set up their plumbing in a standard way
I am not sure sure about that. Even in car assembly, the robots do only some types of jobs. The more plumbing-like aspects (installing equipment and the wiring and tubing between them) involve a lot of workers. And that's in large scale mass production, with the associated possibilities for capital investment, standardisation, optimization. Home construction is already a very different business, and plumbing in existing buildings is yet another step removed from that ideal environment for robots.
I suppose it's technically possible to build a demo-house for robotised plumbing, at huge cost and quite some inconvenience. The history of automation has a lot of such demos, most of which never turn into a solid business case. I bet there's already something like this, somewhere in Japan.