Here is a theory that I haven’t been able to find in the.
how about the reason why Roger laughed was because the one piece is fake. He traveled all this way in search to try and find this one piece and when he reached there he found nothing. Roger laughed from hysteria because, all this time what he was looking for was fiction and a cruel joke so he was laughing not because of what he saw but because of what he didn’t see the, ONE PIECE. So in a final attempt to revive the pirate era he lied and told the world the OP was real so he could keep the era alive and so maybe someone could find the OP since he couldn’t!!!