Why do atheists consider the concept of a creator as invalid?

Why is it that atheists find it more plausible that the intricacy - order to detail - of the universe, the earth's fine tuning of life, the human genome - which in itself has, merely for ONE strand of DNA, a building code immensely complex than human language - and the moral law in man's being, all came as nothing other than a result of chance/random happenings but most difficult to accept it was created by a creator/intelligent designer??