The Flood
Accepting that a global flood did not happen does not negate the possibility that local floods did occur. They did find traces of these things in Mesopotamia. Evidence showed that a terrible flood happened in the past and an ark-like vessel was made to save people and animals. Still, there is no conclusive evidence to say that this did happen in Mount Ararat.
With Faith
The people doubting the accuracy of this flood use scientific facts. For some people, the truth need not be supported by science. The story of the flood and the ark is taken to be an actual description of historical events without any need for evidence. Their faith is all they need to know that the flood happened. Such an approach, of course, would have no place in scientific research.