Because our society infantalises you at 14 when you biologically become an adult and requires by Law you attend high school the first four years of your adulthood. Where a 14 year old from Ancient Times, right through the Middle Ages, and up to the Old West would be able to split logs and build a cabin, use a shotgun to hunt, and be able to get married in the Hills, now they tell you you are a "child". After four years of brainwashing and bullying in the fishbowl social hierarchy of artificial high school, you are so infantalised by 18 that you feel less grown up than you did at 14, so you will buy into needing 4 more years of infantalisation in college. So you push off of a False Premise at 18, the false premise that you have not only NOT been physically mature for four years but are still an infant just floating in mid-air with no real gravity to it, and you go to college until you are 22. Now you're SURE you're an infant and "too young for marriage", eight years into being biologically fertile. By 30 the men all have Peter Pan complexes and women don't think their "Biological Clock" starts ticking until they're 38, but by then no man their AGE will marry them because they have delayed things until they're hitting Downs Syndrome babies by then.