Because good building stone is hard to find in the river valley of the Euphrates River where the Sumerians lived, the Sumerians mostly did not build in stone.
Ziggurats are very high buildings.
Maybe they thought it was better to pray to the gods from as close as possible, and so if the gods lived up in the sky you had to build great platforms to get near them. The Jews thought it was a very bad idea to try to reach all the way up to God like that, and their memory of the ziggurats is reflected in the story of the Tower of Babel .