Why do we not know if any dynasties preceded the Shang

There were no written record of any dynasties before the Shang dynasty, and for good reason--paper was invented in China during the Han dynasty around 100 BC, and the Shang dynasty was said to begin in 1600 BC, a whole millenium and a half before that! A dynasty before that, called the Xia dynasty, has been described in some ancient Chinese texts, but these stories are not trustworthy sources.