The reason why it's dated 300BC-30BC is because it's a sarcophagus from the very last Egyptian Dynasty. Then, they got conquered by the Romans, and most of the old culture started sliding back. Hieroglyphics vanished, the realm was turned into a food exporter, etc. By the time of Jesus, it was still a flourishing Roman Province, it just wasn't what it used to be.
As for shitty mud houses, most of the rural peasantry lived in that kind of housing for pretty much all of history. But they had massively well built cities, the Pyramids (the Giza complex, that is - there's over 200 known pyramids in Egypt) date from 2500BC, for instance...