We don't use it because it would be expensive to make, and modern concrete is stronger (by as much as 5 times stronger!)

One of the reasons modern concrete fails is because steel reenforcement, as it corrodes and expands inside structures. Ancient buildings such as the Pantheon rely on compressive strength for support, so they last much longer. We could build building in this way, but it uses a lot of material and would be prohibitively expensive; especially since most building are not long term investments.