Not all destroyed, and not by flooding. You see lots of similar myths from old civilizations. Doesnt mean those things actually happened. Especially with the differences in the myths.
And the difference in the myths relies with details. Exactly what one would expect to see if different cultures would talk about the same event but from their very own perspective.
So if I saw I saw a flood on the flood plains at 6000 years ago, and someone else saw a flood 5000 years ago in India, you conclude they were the same flood, and happened globally?
We didnt lose anything. We have far better techniques now. It is like the myth that Roman's made better concrete. Just because we we often cheap out doesnt mean we cant.
We have seen such events over a far short time span, I think you are wrong. Natural disasters happen. Especially since the disasters those places had were common to their area.