I was watching Winter's Tale the other day and thinking about this: Why not show average-looking people falling in love in a romantic movie? (Not that it is never done, but it is rare, and tough to pull off, I think for the reasons discussed below.)
Here's the thing: If someone is really in love, the person they are in love with is the most beautiful person in the world... to them. They recognize that the person may not be conventionally beautiful. But the very ways in which the beloved differs from the convention are apt, in fact, to be objects of special affection: his crooked smile, the gap in her teeth, his odd posture, her slightly crooked nose.
But the audience for a romance movie is not in love with the lead actors. And so the audience will not see these characteristics as endearing, but simply as blemishes. The easiest way to convey the beauty that the lover sees in the beloved is simply to cast an actor possessing conventional beauty in the role. It takes true cinematic artistry to make the audience see the beauty of a lover who is, by conventional standards, not especially beautiful.