The most common/popular answer people (mostly men)give to this question is "so that she doesn't look cheap" but the real reason is deeper than that and it's not about not being cheap. Women themselves don't know why they act that way, because the reason is deeply buried in their subconscious, now let's look at nature and delve a little into the female physcology. As far as nature is concerned, the primary aim for physical attractions/relationships between the sexes is reproduction/continuity of the species. Now there must be a way to regulate mating, thus control population, and women/the female species have been selected/wired for this. (N.B the idea of contraceptives and other methods are man made)
This is evident practically and even biologically, we know women have low testosterone secretion, thus low libido when compared with men, although there are exceptions for nymphomaniacss but they are rare types of women. In most, if not all cases, it's women that determines whether sexxx takes place or not. It's not only in humans but living things generally, it's the females that decide. Let's look at practical examples. We have all seen how the rooster chases the Hen in attempt to mate with her, also the He-goats, etc most times their chase is futile as the females won't agree easily and always seem not to be interested. The same applies to humans So, regardless of whether a woman is attracted to a particular man or not, it doesn't stop her from proving difficult/playing hard because that's how nature has designed them, for reasons stated earlier. the only difference will be the yield time While it will take an indefinite length of time for a man she is not attracted to, the time will be limited for a man she is into.