That's the rub. I don't think that those who believe that Jesus was born of a virgin, lived a sinless life, was sacrificed on a cross until dead, was dead for three days and became alive again and went to live with His Father in heaven do disbelieve in a global flood. It's the ones that don't believe all that other stuff that disbelieve that God could literally flood the whole world.
IMO.
Maybe because they don't understand why God would hide the evidence of a global flood.