Because the so-called "G-d shaped hole" doesn't go away just because you want G-d to go away. Man is still a spiritual creature. He can pretend otherwise but his behavior will show it true.
Even atheists don't rebel against G-d, they rebel against the Judeo-Christian G-d. They say "If G-d exists, why did He allow the tsunami to wipe out Banda Aceh?" They automatically presume a good G-d like the G-d of Abraham. They don't presume one of the capricious gods of Rome and argue against him.
They know exactly who they are running from. What they don't know is the love that awaits them if they would only stop - a love that is better than sex.
Shalom.
According to the introduction to one of my prayerbooks, the Talmud refers to man as mav`eh, an Aramaic word deriving from the word for "prayer" and meaning basically "the animal who prays." The author of the introduction points out that man can't not pray, and that even the soul of the thief cries out to G-d for aid in committing a robbery!