I'm too late to a long thread. I didn't read a thing in it. But, I'll give the main reasons in no particular order. These aren't important to everyone. So pick and choose which are important to you.
1) Feeling of true commitment. Living together with someone who won't commit to signing a piece of paper may sound just fine at first, but eventually it does eat at many people. It might not eat at you, but it may eat at your significant other. Why harm him/her needlessly? Studies show that couples living together on average are less happy and have less sex than married couples. It isn't just a rare idea that it might eat at someone - studies show it eats at lots of people.
2) Legal benefits. You can legally visit the other in the hospital or talk to doctors regarding a medical issue. You can legally apply as married on many different types of forms (which is usually a good thing). You can legally make decisions for the other person if they are sick or dead without the need of court battles with the significant other's family. You can legally transfer wealth back and fourth without consequences.
3) Acceptance by society. Many youths don't care about this now, but they may in the future. Having society accept you and your choices makes life smoother and easier. It could be your neighbor, your coworker, your church, your family, your friends, etc, someone will attempt to put you down for living together. It might be nothing more than a disapproving look. But, it might be someone choosing to make your life a living hell.
4) Monetary benefits. Wedding gifts can easilly pay for the wedding and far more if you are frugal with the wedding. Pensions, social security, etc. will pay the spouse as well meaing more money to you or your significant other. Tax benefits, insurance benefits, etc almost always make it cheaper to be married.
5) There are many other benefits, but most of these can also be had by living together, so I won't bring them up. The items above are generally exclusive to married people.
Then lets flip the question around. Why not get married? All it takes is a piece of paper for many emotional, logical, spiritual, societal, and financial benefits. In the worst case scenario, you get divorced and a judge splits up your assets. But that same thing happens if you live together and break up, someone can always sue the other and a judge splits your assets.