Short answer: they are visual mediums and characterization is hard.
On our expectations of actors’ looks–without talking about ‘Girls’ - TV, TV Guidance - Macleans.ca
The main reason for this is that in real life there are many different reasons why people would get together, beyond looks – which, after all, are subjective. But the actors are often playing characters who don’t have any of the redeeming qualities they have in real life. Woody Allen in real life is smart, talented and successful. But the people he plays in films are usually not very smart, talented or successful. (He was most plausible as a romantic lead in Annie Hall, one of the few movies where he really played someone on more or less his own level.) You can believe that the real Larry David could attract someone for reasons other than his money, while it’s hard to believe that of the fictional Larry David, since his bad qualities are so exaggerated.
It’s also very hard to establish any other reason beyond looks why characters are attracted to each other. It can be done, it’s just very hard, and maybe impossible to judge until you see the actors on film together. Writers try to do this all the time; any time there’s a couple, they try to establish some reasons why they’re in love, so it’s not just a superficial physical attraction. And a lot of the time, the reasons are unconvincing: they’re compatible because they engage in “witty” banter that isn’t witty at all, or they both like some poet the scriptwriter vaguely remembers hearing of.
In the end, a lot of movies, TV shows and even stories about romance are basically about physical attraction whether they intend to be or not. Maybe it goes back to fairy tales. In a fairy tale, the Prince and Princess get together in the end because he’s handsome and she is beautiful – there is no other reason given why they belong together, and we wouldn’t believe it if there was any other reason given. Most romantic movies and TV shows are sort of fairy tales, and they have the same thing going: when a man and a woman hook up, it’s because they’re the most beautiful and magnetic people in a world of beautiful people. In real life, when someone who is not conventionally beautiful is attractive, we accept this as a matter of course; different people are attractive for different reasons. But in TV and film, our expectations are founded on that early fairy-tale experience and the experience of watching all those movies that are disguised fairy-tales. It almost seems to require a special explanation, even if it shouldn’t.