Why in the hell would an Atheist celebrate any birthdays?
Because I like cake, and presents, and beer.
They believe they were just a pile of shit created by chance anyway, right?
No, I don't think I'm a pile of shit. As the late, great, Carl Sagan put it: "We are made of star stuff. We are a way for the universe to know itself".
Ironically, it is the christians (and perhaps all followers of the monotheistic religions) that we were made of dust or clay.
Chance is always a factor in this universe. If my parents never met, I would not have been born, bit they did, and here I am. Any measurement of how small that chance is, is useless, since if my parents didn't meet each other, they would beyond any doubt have met someone else, with whom they would have propagated the human speciec.
Now if they wanted to celebrate something they could celebrate the night their momma got drunk, screwed a one night stand and her reproductive cycle hit the freakin' jackpot like a damn slot machine!!
Just because that was how you were conceived, doesn't mean that we are.