Why do we not know if any dynasties preceded the shang

The problem is that we don't know for sure, but we suspect that the Shang dynasty reigned in China starting in 1600 BC, and that before that was the Xia Dynasty.

The existence of the Xia Dynasty is not documented directly, because the first durable documents in China, in the oracle bone script date to max. 1400 BC. Some scholars suspect that the Xia dynasty was created as a myth to provide an opposition to Shang, as their traits were often opposite. On the other hand, we know some potential members of the Xia, which would serve as an argument in favour of their existence.