That is because it's not an "atheist" explanation for anything. It's an
explanation based upon observation rather than on a literal interpretation
of Genesis. Creationist like to tell each other than evolution is merely an
atheist philosophy and not a science. In order for that to be true one must
ignore reality. Ignoring reality in order to keep believing that one chapter
of one ancient book is literally true makes a lot of people--even a lot of
Life is short, but it is also very wide.