I will tell you why, and it is a very simple reason.
There is absolutely no more reason to believe in the bible, quran, etc. than there is to believe in fairies, leprechauns, ogres, trolls, centaurs, cycplops, etc. etc.
In fact, there is far more proof that dragons specifically are more real than ANY god that a human civilization has every imagined. There are stories of dragons all over the world from civilizations which had never even met or had anything in common, and at the same time they all have more or less the same details about dragons, the flying serpent breathing fire.
So a better question for you is this, why do you believe in God, but not fairies or unicorns or vampires? Just a quick addendum, before you start with " Moses and Jesus were real people" so were King Arthur and Robin Hood and maybe even Lancelot and Merlin, why do you not believe in them instead of your God? What makes YOUR god so more special than all the other fiction out there?