Because there was no world-wide flood. There is plenty of evidence for some massive local flooding in the area of the mid-east.
There may be several grains of truth to the flood mythology of Noah and similar mythologies from elsewhere in the ancient Middle East. About 30 years ago it was discovered (" Noah's Flood" by Ryan and Pitman) that in antiquity the Black Sea was a freshwater lake with a water level at least 155 meters (510 feet) below its present level. It was cut off from the Mediterranean Sea by a silt plug in the Straits of Bosporus. This plug broke through about 7600 YBP due primarily to the dramatic rise in sea levels caused by the melting that ended the last ice age.. It created an immense waterfall whose sound was most likely audible for 100 or more miles. The Black Sea basin filled to its present level over a period of several weeks. It is estimated that the shore line advanced at the rate of a mile or more per day. For the people living around the lake it was a catastrophe of immense magnitude. It was likely the single most memorable flood in all of human history. The racial memory of this event probably inspired the Gilgamesh epic which in turn inspired the Noah narrative in the Bible. The evidence for this flood is scientifically solid. This prompted the National Geographic Society to finance an underwater search along the ancient shoreline for evidence of pre-flood human habitation. This search has been successful! A settlement has been found at a depth of 90 meters approximately 12 miles off the coast of Turkey. It is in a remarkable state of preservation because it is located in an area of the Black Sea where the water is completely devoid of oxygen with the effect that biological decomposition does not take place. This means that wooden artifacts such as tools, planks, housing beams etc are preserved intact. What is also quite amazing is that while there is solid scientific evidence for this local flood some 7600 YBP, there is no evidence at all for a worldwide flood just 4300 YBP. One would think that a more recent, more catastrophic event would have wiped out evidence of the earlier Black Sea event. There is also evidence for a similar event causing the flooding of the Gulf of Arabia about 10,000 YBP.
