This all happened just a few thousand years ago. Although there is no common agreement about exactly when, most Bible literalists put 'creation' somewhere between 6,000 and 10,000 years ago and the flood somewhere between 4,000 and 6,000 years.
So, if our assumption is true, we would have had the entire population of Earth descended from this one surviving family all of whom were convinced and devout believers in the 'one true god' - and nothing else. They would have seen the world exactly as the Bible describes it, no more and no less. What then should we expect of their descendants of just a few hundred or maybe a thousand years, around about say, 3000 BCE?
What we actually see when we look at the evidence, is Earth populated by people not only differing considerably in physical appearance and from DNA evidence, showing signs of considerable genetic diversity, but, most importantly, we see Earth populated by people with entirely different languages, religions and cultures, and even entirely different beliefs about their origins, some living within a few days journey of where this all supposedly took place.
Not only have all other people, apart from one small Middle Eastern tribe, apparently forgotten all about Noah and his family, they have also forgotten all about this 'one true god' and his awesome powers and have invented whole pantheons of different gods in many cases. Some worship the Sun and/or the Moon. Some have local gods of water sources, woods, volcanoes, natural phenomena such as thunder, rain and/or wind. Some have male and female gods marrying and producing daughters and son gods. Many of them also have stories about these gods speaking to them, manifesting themselves in different forms and definitely answering prayers.
In fact, as Europeans explored the world from the early Middle Ages onwards, never once did they find a new group of Noah's supposed descendants (if we are to believe the Bible) who actually remembered any of this supposedly true history in the Bible. Not a single person outside the cultural sphere of influence of Europe and the Middle East, and even many of those with a long history of contact, had the same religion and the same set of stories of their origins. Not one!
None of them remembered Adam and Eve or 'original sin' or used it as a reason to treat women as inferior to men or to make humans the owners of the Earth and everything on it. If they held these beliefs at all, they had invented other reasons for them.
So, if the Bible is true we have to explain why everyone else throughout the entire world forgot every detail 'known' to Noah and his family with such absolute certainty, yet just one small Middle Eastern tribe remembered it in absolute detail, including, word perfectly, exactly what was said to whom by whom, where and when.
Since the god of the Bible, if we are to believe it, took steps to ensure the Hebrews remembered this all with such perfect clarity, presumably it could have ensured everyone else did, but chose not to. Maybe this mass amnesia was all part of its plan - which raises the question of why it planned to have everyone else forget apparently so they could be told about it later.
If it created humanity with poor memories so religions and origin myths would be forgotten within a few generations and needed to be randomly reinvented, why has it apparently reversed that now and religions and origins myths appear to be retained for many hundreds or thousands of years, no matter where the believers migrate to provided they do so in sufficient numbers.
Having allegedly allowed 'evil' to enter the world, then having tried to eradicate it and failed despite being omnipotent, the god of the Bible seems to have facilitated the spread of the 'evil' of the 'wrong religion' and then had to have a book written to reverse this mistake, but has still not yet managed to convince a majority of the world's people. Presumably, Bible literalists can explain this apparent incompetence.
Presumably, Bible literalists not only accept that there is a plausible explanation for all this but can suggest a reason for this mass amnesia which appears to have afflicted the entire human race about 3000 years ago save only for a small band of Middle Eastern hill farmers.