Was the idea of life after death prominent in the days of multiple deities? Those fanciful deities were artistic expressions for something indescribable.The gods were said to be immortal but humans, not so much.

With the development of the One-God philosophy, unity was realized. I am one with Brahman, timeless. It’s not that there is a here-and-now and later an afterlife—it’s all the same eternal now.

Because there is no evidence for either, they are both invisible. And since humans are animals with pattern seeking brains, who like to classify, if two things share a quality, then they both belong in the same box.

Or. Since there is no profit to be made from post-death existence for anyone, especially priests etc. , unless you can make the claim to be able to exercise some control over it. Then you need an employee, ( God ) who can manage the technical details for you.

Maybe because we’re biologically predisposed to believe both?

Because they want everlasting happiness after a life of suffering and imperfections?

I don't know either. I certainly dont. it's a matter of science to me.