I don’t know how I know the answer to this, and I’ll try to keep it brief and simplify something more complex than I can go into given I’m typing on my phone, but here goes:

This goes wayyyyyyy back into our past as human beings. Women largely vocalize more than men during sex as a way to signal to other males in the area that sex is happening and that they should join in. It’s encouragement for the male, but it goes deeper (heh) than that - the human penis is quite large as far as body to dick ratio for animals goes, and there are two reasons for that; the head is designed to ‘scoop’ competing males’ semen out with each thrust in preparation for replacing it with the dick-haver’s own, while the longer shaft allows deeper penetration in order to scoop as much as possible.

So basically, when a woman moans loudly, it’s signifying she’s ready to go, and that the strongest male in a group will be the one to eventually impregnate her. It’s literally a survival of the fittest mechanism.

Now obviously that isn’t the reason for it these days, as we’re all aware that our intelligence as a species makes sex a vastly more complex thing than it is for other animals at this point in our evolutionary path, but that’s what researchers believe is the origin of sexy female noises.