Something an old-timer told me that rings true: women marry men for their potential, men marry women for the actual.
Guys hear that women change once you marry them and that's their greatest fear. Women stop doing all the things that they used to do when they were trying to get you and keep you. Then, once the wedding takes place, all those things stop. I've heard from many men; as soon as you get married, oral sex stops, then regular sex gets less and less frequent and your woman stops doing all the things that made the relationship special. If that's the case, what's a man's incentive to get married? A man already knows that it puts him at a disadvantage financially to get married if a divorce occurs. 95% of the time, if there are children involved, a court will dictate how much of his income goes to his now ex-wife to care for their kids (wife almost always gets custody, so why even fight?) He can only see them when she allows it and if she doesn't, the courts won't punish her. Going to court means retaining a lawyer, which costs even more money, so now you have to get dictated to by your ex-wife on how you deal with your children.
I know that this all sounds really bad, but these are the things that go through a man's mind in that situation. So for him, marriage is a poor risk. Why mess up something that seems to be going so well?
On a personal level, I've been with my wife for 26 years, married for 20 and have 4 kids ranging in age of 8 to 21. Why did we make it? Because we realized that the more work you put into a relationship, the more you'll get out of it. Communication is the key. People are way too selfish to have the marriages that our grandparents had.