ITA! It's very similar to the way romance novels are constructed, I think, in that once the couple says "I love you" or works out some sort of commitment plan, their story is essentially over, so the HEA stays intact.

Maybe it's a lot harder to write good, functioning relationships with believable conflict to keep it interesting than UST? Or stories tend to be so UST-centric that other types of relationships end up shoved aside or maybe unmarketable or something.