We have gotten off topic.
The question is Why does the Bible describe a universal flood if there is no external evidence of this flood?
In my opinion the answer is history. Conservative Christians want to believe that God wrote Genesis and dictated it to Moses or whomever. There is no real evidence that this is the care.
History is not the past, but how we understand the past. Genesis is historical in that it is the result of the Hebrews attempt to understand the past.
How do historians know what happened in the past? They begin with the best available sources. The best available sources said that there was a universal flood in the distant past. Furthermore these sources were imbedded in the culture of the Semitic people of which the Jews were a part.
Thus the Flood was a part of basically pagan culture from which Judaism developed, so it was an accepted fact of history. The Jews could not just say that science did not verify the story of the flood, because there was no scientific info at that time. The question was not: Did the Flood happen?, but how the Flood did the flood take place?
Thus it was the task of Jewish historians and theologians to give a Jahwist understanding of the Flood, different from the pagan understanding of the Flood, which they did. In other words, Why did YHWH create the Flood? and why did YHWH save Noah and his family?
Does that make the universal Flood real? No.
Does that make the story that we find in the Bible true? Yes, to the extent as the historians correctly grasped through the power of the Holy Spirit how the God of the Jews YHWH could have acted through the Flood.
Conservative Christians often say that Genesis is a “historical book.” That is not true if one means that it is a story of exactly what happened. It is historical in a much more meaningful way in that it is our understanding of how God/YHWH relates and works for God’s people in the real world of history as best we understand it.
This also true of the beginning of Genesis. We have to remember that the Bible was not written for 21st century Western folks, but first and foremost for the Hebrews and Jews who wrote it. It is not the facts of history that make it true, It our understanding of God’s presence in history that makes it true.
We accept it by faith, and not by sight, until God shows a more meaningful way to understand it.