No one claimed we say it all came by accident. It's the creationists who claim that we claim that.

There only had to be one first self-reproducing cell at the beginning, and natural selection did the rest. We don't know how that cell came into being, maybe it even was an accident, but if we look at the size of the universe, which is propably even infinite, then that's not impropable at all. And if the universe really has an infinite size, the chance for anything impropable, but possible to happen somewhere in the universe would be 100%.