That is a great question and one that is not asked often enough. Unfortunately, we have a lot of conflicting data to our current understanding of human civilization origins. Our current paradigm says that many cultures that were supposedly not connected in any way developed very similar technologies, which defies logic. For instance, many megalithic sites around the world have common building construction techniques that cannot be denied and their inhabitants were halfway around the world.
There are a ton of anomalies that don't fit our current paradigm. There are those who have a vested interest in maintaining the status quo, who may have based a good portion of their professional lives on developing theories based on previously 'discovered' information. But I think, more realistically, it is an expected response to discount anything that would contest one's understanding so engrained. If someone accepts that our understanding of the past is vastly different than what we are taught, they would then question many other things. I don't think most people's psyches will take them there.
The ancient Egyptians had corn depicted in heiroglyphs and yet corn was only native to the Americas. How did they get it if they supposedly weren't able to traverse the seas? Most people are not taught that there are pyramids all over the world (not just Egypt and Central/South America) and yet there is no knowledge passed from one culture to the next? There are stone circles throughout the world with solar, lunar and constellation alignments. They can't be dismissed as a natural progression of a primative people independent of each other. Anomalies by the bunches that don't fit into the current paradigm are usually dismissed as probable hoaxes without investigation. That's not to say that hoaxes don't exist. Gobekli Tepe, a "temple" found in Turkey predating Stonehenge by 6,000 years, upends our understanding of human civilization origins and yet most people have never heard of it.
It is highly improbable that civilizations can develop independently of one another and yet share common technologies and develop similar beliefs. Once we accept this, then real understanding can progress.