First of all that's because they're non-combustible and may take very high temperatures. the essential function of a fireplace is to contain a fire, which implies it's to be ready to stand up to high temperatures (repeatedly, and for long periods of time) while not burning, melting or cracking. Right away, you are restricted within the variety of ordinarily accessible materials you'll be able to use, however it also needs to be price effective, and durable enough to create a structure out of while not it breaking or collapsing the first time you encounter it. the only ordinarily accessible materials I can think of that meet all those necessities are unit metal, bricks and rocks, all of that fireplaces are ordinarily made of. Rocks could also be less common, just because bricks are each cheaper and easier to make with.
Hope it helps!<3