Romantic comedies are (arguably) the most formulaic of all movies, and studios use that rom-com formula to make a bunch of money with very little effort.
Many fans enjoy the predictable nature of rom-coms, and will actually get upset if a film strays too far from the norm, so it's no wonder filmmakers don't even try to do anything different with the genre.
In fact, romantic comedies are kept predictable right down to their poster layouts, of which there are exactly around five types.
There's the Back to Back, the Two Guys One Girl, the Pick-Me-Up, the Men Are Inept and the Well, Here's Everyone!!, five poster types used to advertise the less than five types of romantic comedy plots.
The list is missing one overused type of rom-com poster- the Close-Up of Guy and Girl Making "Funny" Faces, a true classic!