Why?  Because humans did not understand how the world worked.  Natural events like an earthquake, flood, or volcanic eruption are well understood today, but in ancient times beings with souls (Latin animus) were thought to be causing these great events for a reason.  If an earthquake occured and destroyed a city, humans assumed that somebody with the power to shake the earth (god) was upset at the city.  Thus, they should worship this god to make sure he doesnt do it again.

Its only natural for humans to want to understand their surroundings.  When humans knew very little they attributed everything to gods.  Now that humans understand their surroundings more we no longer need mythological figures to explain things.  We still try to explain things though, its our nature, read into the Steady State Theory (countered the now accepted Big Bang Theory), Dark Matter, String Theory, etc. for humans trying to explain things they dont fully understand.

Bablyonians were around before the Hebrews, so historically and logically the Hebrews copied their religion from the Babylonians.  The Enumu Elish (sp?) even goes further than the Bible to explain in detail what the world was like before the creation of Earth, specifically the war between the good and evil gods (God and Satan) that is in Judeo-Christian mythology but barely mentioned in the Bible.