In the 80's just the title of a teen film gave away the whole plot of the story. It was so obvious as to what was going to happen by the end of the movie even before watching. Yes, these movies are obvious, but they are the classic stories. These movies work in any decade. Whether obvious or not people want to see their dreams and fantasy's come true for the characters on screen.
Film makers know what it is that teens are willing to pay to see.
Even though the times are changing, people are basically the same.