Only because they have left so much written record that we don't really expect that another record will tell us something we didn't already know. The less we know about some culture the more valuable written record would be - but often it isn't available. When we already know a culture well archeology can tell us where they ideals they wrote about themselves don't match reality. When we know nothing though, archeology leaves us with a lot of evidence that we don't really know what to make of.