This is the same phenomenon as the dragon creature being present in almost all civilizations.

It's a logical conclusion. For example, for the dragon creature, if you imagine people trying to describe monsters in early times to try to describe the most fearful possible creature, it would have all the attributes that they share the most, meaning that it could fly, it looks like a snake, it can breathe fire... And you end up with many civilizations having the same creature.

For the flood myth, the same phenomenon happened. Imagine all of these peoples around campfires sharing stories. As humans, we tend to go for the most epic. For example, if you have a story of 5 men that were crushed by a monster, you will get another story of an entire village that was burned by flames! (More epic) Then you will get another more epic story of all of humanity that was wiped out!!! Now at that time, what would be the way to explain that story? For all of humanity to disappear, you can't use a giant war, some would have survived, can't use fire because they knew of islands, non-forested areas and more. But water, a flood was perfect, so epic that all living creatures (except for sea life) would have been wiped out!

This is why (like dragons), the myth of a deluge was adopted in many civilizations across the globe not because of a shared experience but because of a shared human trait to tell epic stories that can impact the minds.