Why do we still say “bless you” when someone sneezes?

It’s tradition – and seeing as we live in a “christian” society, it’s considered the norm, I’m an atheist, and i still say it, i think people will think your rude if you don’t I read a book recently by Penn Jilette from Penn & Teller – and he’s an atheist, and isn’t bringing his kids up religious so amongst other things brought them up to say “How Funny” when someone sneezes. I can’t think of anything better to say, someone should start a facebook group to get everyone using this