Historically, when a house was built, the plumbing was run under it. That makes a lot of sense since the water lines are in the ground. Before concrete foundations, the pipes were easier to get to. You just needed enough crawl space under the house to make repairs and additions. For some reason, the plumbing was still put under a house when concrete foundations came into being. Now a jackhammer and a good home owner’s insurance policy was needed to get to the plumbing. The location of plumbing in new construction saves on hassles. Basically, engineers figured out that it made more sense to put the pipes in the attic.
Pipes in an attic are much easier to get to. No crawling under the house. Attics usually have more head room. Repairs go faster and save on frustration. During construction, pipes are run to the location of each bathroom, kitchen and other water source. They are brought down the walls, between the frame studs. Even though sheet rock or some other material will eventually cover them up, it’s very easy to locate them in the future. The guesswork of wondering just where that pipe is, under the floor, goes away.
Beyond construction considerations, there’s the maintenance situation. Pipes in an attic are much easier to insulate and protect from cold weather. Granted, pipes under a slab won’t freeze, but pipes under a pier and beam house will. The only other maintenance issue that can arise, if the attic is the location of pipes, is the possibility of one breaking. That’s a big problem. That water is coming right down the walls, and it will damage sheet rock and flooring. Hopefully, that would never happen, but it can. Just remember that it’s easier to check on pipes located in an attic than under the house.