Sometime it could be because they have stayed together long enough to have gained the wisdom that tells them "Just because I'm unhappy today with my spouse doesn't mean I will be unhappy with them forever".
Because that's the truth. Ask people that have been married for long periods of time (20 years or more) and most of them will tell you that they went though some bad times. Things can get difficult and hard, cares of life can work to pull you apart but if you hang in there the hard times won't stay forever and it can be very good again. I don't know how you tell the difference between a marriage that is irreparable and one that isn't minus cheating, beating and things on that scale.
Committment will hold you together when love won't. Love is an emotion that can ebb and flow. Committment is a decision.
Personally, I love my dh more now than I have at any point in our marriage. But there were bad times, especially when we had a little one, lots of bills and not so much cash, stressful jobs, family issues and a thousand other problems. But we hung in there in the rough times and when I look back I know our good times have far, far outweighed our bad ones.