My biggest question would be; how do you believe in "luck" but snub superstitions. Luck is a type of superstition. Who had the rabbit's foot when The Big Bang took place?
As others have already pointed out, atheists believe in luck because we can see it happening. Significant events, good and bad, happen to people all the time. Sometimes they can be attributed to deliberate action, sometimes random chance. Before humans existed, it happened to our distant animal ancestors, and before them to nonliving things. Causality can be extended right back to the Big Bang with no problem at all.
Either way It was best said by the Theoretical Physics professor that narrated the How the Universe Works, Expanded episode I watched yesterday. There is much more to learn and the universe is still a mystery. He says this at the end of every episode. That is the only real truth.
Few atheists would disagree with that. I will point out, however, that so far empirical evidence supporting a universe-creator is zero, and evidence that the universe supports itself
is quite abundant.