Indeed, but clay tablets could have been thinned, they could also have used organic material (with fired pieces it burns away, cf paper-clay) to lighten the resulting tablets without much loss in ruggedness (paperclay is stronger than clay alone, like concrete vs cement).
I think such a development would lead naturally to think "damn this epic of Gilgamesh needs a few camels to transport, perhaps instead of impressing back in to clay I can press it on something lighter" and then possibly with the realisation that pressing on a damp cloth with an unfired tablet impression produces a great serviceable print ...
