Muslims once translated many ancient Greek texts. Could it be that much more is preserved, but not available to the international historical community? Could they be kept in mosques in Saudi Arabia, or have they been searched by westerners during the colonization of the arab world after their defeat in the first world war?
Well I can't possibly find hidden archives using Google but there are ancient archives. And hopefully this would be a relief to you in some way. And I'm community wiki-ing this answer as this is only a partial answer right now.