Marriage is not something a lot of college students think about regularly, although many students do get married before they graduate; some even have kids. However, once upon time the tradition was to at least be engaged by the time you graduated college, maybe even high school. My own parents met while in college, and for a lot of my friends' parents, that seems to be the norm. These days though, it isn't unusual to graduate college unattached.
Today, college is seen as the time when you should meet new people and try new things. "Hooking up" with someone even though no emotions are involved is not that unusual. Couples may date for a long time, but that doesn't necessarily mean that they will get married.
I have a friend who once said she expected to be engaged by the time she graduated, which for her would have meant 21. That just seems so young to me. It certainly is comforting to know who you will marry when you graduate; it will help you make all kinds of decisions that before you may not have even known where to start. Where to live is one such decision. When another person is involved in making the decision, that limits your choices considerably. Not to mention, you know you'll have a roommate that you trust and can split the rent with.
At the same time, that wide range of options may be exactly what you are looking forward to. Graduating can be an exciting time, and without another person in the picture, your choices are endless. Joining the Peace Corps is not really an option when you are engaged or married, but it may be the perfect choice if you are single.
The dating scene after college is confusing. In college it was easy enough to meet people and you had plenty of chances to get to know them. But what about when you are working or in grad school? You are no longer around just people your own age whom you are likely to have some things in common. Some people find it comforting to know they won't have to worry about that.
It's a choice that you cannot know how you will choose until you are faced with the choice. Few people expect to be married or engaged while in college, and those who do may be wrong.