Because it is my understanding that you refuse to believe in the power of things that you cannot see, so if there was a shark 50 feet underwater, you might ignore all the warnings and go swimming anyway.

Then you might get eaten and become an evolutionary failure, and that could be embarrassing since you're so into evolution and all.

Can you picture that? An atheist standing on a diving board and everyone's shouting "wait there's a shark", but the atheist just yells back "it doesn't exist" as he leaps to his fate.