A huge part of the value of college its option value: i.e., finding out (a) is a diploma is worth it for me? (b) what kind of diploma is worth it for me? Even if the answer is that the student should drop out and do something else, going to college is valuable because it helps the student figure this out.
Do high dropout rates imply that college is a bad investment? No! You can't determine whether college is worth enrolling in by looking at dropout rates. To the contrary, 46% dropout rates are good evidence that people are learning a lot about the suitability of college for them. If they knew up front that they were going to drop out, they might not bother attending in the first place.
College is a great facilitator of information flow. It is a chance to efficiently learn about a great many things. You learn about subject material, yes, but also about your own affinity and aptitude for various subjects. You learn a lot about yourself, and about other people, too. It's relatively easy to find friends (and more) in college.
Making the best use of college is a tricky business. For one, it's too easy to take college for granted. College campuses dramatically lower the cost of interacting with a variety of subjects and people. In fact, because they are part of a trend of decreasing costs in life up to that point, it's natural to think the trend will continue for years to come. And then you might not focus enough on making low-cost investments in your future, instead of living it up in college.
The college atmosphere can trick you into thinking it's a good idea to major in something that won't help you after graduation. There are few expectations imposed upon college students, which is how they come to have so much time for exploring. In such a low-cost environment, nearly anything has a positive return. The real world, by contrast, expects people to do stuff that generates value for other members of society.