In many cases, when archaeologists reached a tomb, they found that it had already been extensively damaged by robbing. It is also said that these robbings did NOT take place in the modern era. Who were these premodern robbers who had the knowledge to know where the tombs were and how to get inside them?
There's always more to be said on the topic, but I touched on this in Who were the grave robbers of ancient Egyptian tombs? and this SASQ post.