Why build an Ark?

[no one]

gawd really likes sea men.

[Gawdzilla Sama]

What I do on the weekends is MY business.

[Ferrocyanide]

The point is that these stories endured, not because they were great fiction, but because they were good entertainment.  Though I suppose if you're competing with a tale of wandering the desert for 40 years thanks to an early version of Mapquest, a sort of "Lord Of The Rings in reverse" is not the most promising.

I agree. Life was hard. There was no TV, radio, books, internet, cinema.
People need entertainment in the form of jokes, in the form of stories.
So, someone (the author) took inspiration from somewhere and told a tale and another author took the story and modified it and so on.
Eventually, it was inserted into various religions and considered as fact.
It as difficult for people to tell what is fact and what is fiction, back then.

As for the flood story, efficiency is something measurable.
The jewish god is not being efficient and we can conclude that he likes entertainment. He doesn’t take himself or anything that is happening seriously.