Why were fired bricks first used mainly for pavements and exposed wall tops in Mesopotamia?

In ancient Mesopotamia, the production of fired bricks was a labor-intensive and fuel-expensive process compared to the use of traditional sun-dried mud bricks.1

Because of these high costs, fired bricks were initially reserved for specific structural applications that required greater durability and resistance to environmental degradation. They were primarily utilized for pavements and the tops of walls, where the material was subject to significant wear or direct exposure to weathering.1