Why didn’t Adam pass on his knowledge about G-d? If Adam passed on his knowledge, there would not be a doubt about His existence, and there wouldn’t be a generation like Abraham’s, to where he would need to figure it out on his own.
Adam obviously passed down his knowledge and experiences, as every parent would and does, to his children. Nevertheless, the nature of man is that with the passage of time and generations, things get forgotten, or perspective is lost (or thought to be lost), wears off, and the effect gets weakened. Just look at the Holocaust victims and their children, as compared with their great-grandchildren, and this is even in modern generations with videos, tape recordings, and pictures! By the way, in the ancient world, there was no doubt about the existence of God (especially in Israel where there was prophecy and miracles, but even among gentiles) , just about His identity, His nation, etc.