Because they've been sold on the idea that they can delay becoming a fully functioning adult for few more years. They've been convinced that attending college is a goal, rather than a means to accomplish a goal. Actually getting that education and preparing oneself for the world beyond campus is secondary to the experience of college. Growing as a person so one can become a compassionate, caring member of the global community is the reason to be there, not learning about organic chemistry so one can become a better, more competent pharmacist, which is much harder.
The colleges would rather sell people on the experience of college over the education of college. The experience is much easier to sell and requires much less effort on everyone's part. The government (via the taxpayer) is footing the majority of the bill (via guaranteed loans), so everyone wins.