Why do so many people in the U.S. marry so early?

Maybe it's just a misrepresentation but I've read this sooo often on internet forums with a lot of Americans "my wife is 27 and..." or "I've been together with my husband for 6 years now, we're both 24..." etc. The terms "girlfriend" and "boyfriend" only seem to correlate with people under the age of 21 or 22. Also, I've lived in America for a long time as a teenager and I know quite a lot of old high school friends from those days who are already married now (I am 25 years old). I find this a bit peculiar to be honest. I've been in a long-term relationship for several years now but I don't think I would want to marry just yet (and certainly not at age 22 or 23). My girlfriend thinks the same. My girlfriend is from east Asia where people usually marry relatively late. The factors that are generally considered important are: finish university, have a secure job and live on your own - then it's acceptable to marry. This usually happens when people are between 28-32. In western Europe, where I am from, the whole thing is more liberal (similar to the US). There are no "cultural rules" that you should adhere to in order to make your parents/relatives happy. If you want to marry while still in college, that's completely okay with most parents. However, for some reason almost noone chooses to do that. Young people usually just end up marrying around the age of 30 anyway. When I ask my friends (also those in serious relationships) if they'd like to marry soon, they all say "hell no! I wanna have fun... be crazy... there will be more than enough time in my life to be married!" Really the only people who end up marrying very early on in Europe are members of sects or just extremely religious people. But when I look around in the U.S., it seems like so many completely average people get married so early on. How come?