This is the story that I have heard….. Piping was always under the crawl space or in the basement, so when construction shifted more and more to slabs it seemed obvious to leave it there. Lessons were learned.
In some cases copper water pipes can still be put under the slab but they have to be wrapped in plastic or run in conduit and isolated from building movement. It is supposed to be silver brazed, instead of soldered or one piece (without joints). Islands with sinks require pipes underground for instance. It is put in conduit and should be copper not the increasingly popular plastic stuff.Depends on the jurisdiction and inspector.