You make a valid point. Couples who lived together for 8 years and then got married and divorced 2 years later are remembered for having a marriage that lasted only two years instead of being in a monogamus relationship or 10 years.
Personally I have known couples who have been married for over 25 years (on paper) but live in different quarters of the house or not even in the same house. They attend family functions and each other companies' holiday parties....etc And then there are other marriages which are "sexless" where couples are little more than roommates with the same last name, companions, and friends. Surly no one would suggest a young couple in love ask any of these people for the "secret" to marriage longevity.
Several years ago I heard a woman on the radio ask her grandmother who was coming up on her 50th wedding anniversary, "What is your secret?". The elderly woman paused for a moment and then she said, "Baby, we just stayed together." We as a society do seem to believe that couples who stay together must do so because they are "happy". However like you stated there are numerous reasons why people don't bother getting divorced.
Some people don't have any "deal breakers". They will stay together through cheating, lying, verbal/physical abuse, alcohol/drug addition, and constant financial turmoil. If staying together through all this means "success" I'd rather be known as a failure! lol!