It's the same reason why adult love can't be like one's "first love" in Jr. High School or even college for that matter.
Movies and teens don't focus on all of the worldly stressful tensions people have to deal with. (An overbearing boss, mortgage/rent, car payments, squabbles with exes from previous relationships/marriages, children issues, and countless other obligations which battle for "mindshare" or attention in one's life.
Human beings make mistakes! This also often includes choosing the "wrong mates" for ourselves. We make commitments during the "infatuation phase" of a relationship without truly knowing if we share the same values or want the same things for the relationship.
Most movies end with "the wedding". In reality it's the day afterwards where the marriage (begins). Another thing worth noting is most people don't want to pay $10-$15 U.S. dollars to watch a movie that (doesn't) have a "happy ending"! You're not going to see many "blockbuster" movies about divorce, spousal abuse, and heartache unless in the "end" things turn around for the main character.