It’s because they’re worthless.
A good dad must possess two qualities: the ability to acquire and accumulate resources, and the willingness to invest them in her and her children.
Giving women things that are worth something doesn’t count, because it’s still useful to the man. What matters most in courtship, in terms of gifts, is giving women pretty stuff that has no worth whatsoever. This illustrates to the woman 1) that you can afford to do so, and 2) that you are willing to do so.
Then you get nookie. ::