> Might have been their only way of hardening the clay walls. We do it still to this day, except we burn the bricks before building the walls. Lacking modern building techniques like that
That's not exactly a modern building technique. Ancient Mesopotamian wall inscriptions sometimes brag about how the bricks in the wall have been baked. (Making for a higher-quality wall; presumably sun-dried clay was still in use for non-prestige projects.)
You burn bricks before they are assembled into a building, not after. In the American Southwest you do encounter prestige buildings from the end of the 19th century proudly proclaiming they were built in brick when the rest of the village was built from adobe.
The firing is energetically expensive, and doesn't really end up creating a notably longer lived result. Things last in the southwest because of the low moisture levels, whether they are fired or not. The "prestige" of brick in the southwest was all about having the money to pay for fired bricks, not really their superiority as a building material.
That's not exactly a modern building technique. Ancient Mesopotamian wall inscriptions sometimes brag about how the bricks in the wall have been baked. (Making for a higher-quality wall; presumably sun-dried clay was still in use for non-prestige projects.)