All those things are huge, massive structures... And very, very, very badly damaged by two thousand years of exposure.
I believe that they took one of those obelisks out of Egypt and put it somewhere in the United States (Washington, D.C, or maybe New York City), and the weather there caused much more erosion after only a few hundred years (if that) than the weather in Egypt caused after several thousand years.
Sand does not erode stones that are harder than quartz, and hot, dry, sandy areas preserve more than they destroy.
Besides, Zul'drak wasn't at the constant sub-zero temperatures that would be necessary for preservation, as evidenced by the aqueducts and running water throughout the zone. That liquid water causes much more erosion than sand does.