Why do adults believe in an obvious fairy tale? Because, it isn’t obvious to them. The facts are there, but they’ve never actually been laid out for them. Here are the facts, laid out:
And, of course, there are the mathematical problems concerning the seer quanity of water and the quantity of animals that would have needed to fit onto a single vessell, the historical problems of a culture that (supposedly) built the great sea going vessel ever and then totally forgot how to build large ships entirely, and the archeological problems of evidence for continuous uniterupted civilizations all over the earth.
The Pope himself said that literal creationist belief in the Bible aounts to nothing more than superstition.
For any readers that still ask, “but were does the Bible come from, then” I would suggest Who Wrote the Bible by Richard Friedman and The Bible Unearthed by Israel Finklestein.