People want to believe that doing good things like saving the day will deliver a consistent reward; the idea is an easy sell in movies and is pretty much expected as part of a happy ending. The trope of women as rewards speaks to the desire for the universe to be 'fair', as well as to the conventional idea of happiness. The male hero has accomplished something, and will get the love of a woman in return. As for why women are rewards for men, but rarely the other way around, I think that's some pretty standard Hollywood bias. A lot of the time female characters aren't fleshed out as much as male characters; this is so they can function as objects to move the plot forward/later resolve it, rather than complicate it with their own actions/ambitions/choices.
It happens in films aimed at women, too, which is why I hate romcoms. Legally Blonde is an exception to the rule though if memory serves; she sets out to 'get' the perfect guy through her efforts, and realises that she's actually too good for him (she still falls in love with someone else by the end, because it wouldn't be a happy ending without a wedding lol
, but that's allowed).