Because in that time it was all they believed, so it was not "mythology". Also, the city-states of Greece were so separated they reffered to themselves as "Spartans" or "Minoans" not "Greeks". Im not...
its really all their beliefs, to help people realize how life works- like how the trees grow, how fall works, when to harvest crops, how we get mail, all the basic things in life. we have science...
The tales say that over millenniums of time rocks, planets, etc. crashed together creating the greek titaness Gaea. The dust shot up into the air making a male named Uranus. They populated children...
Greek mythology actually was a relegion in ancient times, it was not called mythology though. It was created by the greeks to explain natural occurances or things that they could not otherwise...