Atheists say that there is no evidence for God therefore they are certain that he does not exist, but that's a fallacious argument.

Indeed it is, but most atheists do not take the strong position "I am certain there is no God." Most atheists simply do not make the assertion "There is a God." There is a difference. most of us are "de facto" atheists as described on Richard Dawkins' excellent "Spectrum of Theistic Probability". 

From Wikipedia:

In The God Delusion, Richard Dawkins posits that "the existence of God is a scientific hypothesis like any other." He goes on to propose a continuous "spectrum of probabilities" between two extremes of opposite certainty, which can be represented by seven "milestones". Dawkins suggests definitive statements to summarize one's place along the spectrum of theistic probability. These "milestones" are:^[2]

  

1: Strong theist. 100 per cent probability of God. In the words of C.G. Jung: "I do not believe, I know." 

2: De facto theist. Very high probability but short of 100 per cent. "I don't know for certain, but I strongly believe in God and live my life on the assumption that he is there."

3: Leaning towards theism. Higher than 50 per cent but not very high. "I am very uncertain, but I am inclined to believe in God."

4: Completely impartial. Exactly 50 per cent. "God's existence and non-existence are exactly equiprobable."

5: Leaning towards atheism. Lower than 50 per cent but not very low. "I do not know whether God exists but I'm inclined to be skeptical."

6 De facto atheist. Very low probability, but short of zero. "I don't know for certain but I think God is very improbable, and I live my life on the assumption that he is not there."

7: Strong atheist. "I know there is no God, with the same conviction as Jung knows there is one."

Dawkins argues that while there appear to be plenty of individuals that would place themselves as "1" due to the strictness of religious doctrine against doubt, most atheists do not consider themselves "7" because atheism arises from a lack of evidence and evidence can always change a thinking person's mind. In print, Dawkins self-identified as a '6', though when interviewed by Bill Maher^[3] and later by Anthony Kenny,^[4] he suggested '6.9' to be more accurate.