This reminds me of a passage from Bryan Caplan's book, "The Case Against Education." Paraphrasing, there is nothing stopping you from walking into a lecture at Harvard or MIT or any other renowned center of learning. No one will ask you for identification or lock you outside. You can buy or download the same textbook and fill out the same homework. (In the case of your "socialization" value, again, no one is stopping you from just walking into a campus and socializing with peers there)
No one does it because we intuitively understand that there is no real value in doing so; the purpose such institutions serve is credentialing, not education.
Well, I did it :-) I did 2 or 3 years of philosophy courses at Sydney Uni. I mean, I just turned up. At the beginning of each semester I'd start doing every possible course, then just keep doing the ones I liked, with interesting/good/admirable lecturers, which turned out to be a lot, 2 or 3 times the amount of lectures weekly than if I'd been enrolled. Also was on their mailing list so also went to special visiting lectures, a regular discussion group, etc etc. No exam stress! I had such a great time. But yeah, most (non-university) people I mentioned it to laughed as if it was crazy.
Most people don't have the time freedom to be able to do this. The opportunity cost would be too high, as they need to work to eat, and sitting in a class isn't work.
Socialization is a huge part of it - perhaps the biggest in terms of impact - because students who worked alongside each other during a similar stage of life, living within short walking distance of each other 24/7, working toward a similar goal, develop a bond which can be immensely valuable later on in networking for job opportunities.
Those students are much less likely to develop such a bond with some rando who just walked in off the street to check out what's going on. That person has zero investment in the experience other than expressed interest and would I'm certain be measurably less successful forming relationships with other students who could help their career in the future. They aren't in the same tribe.
It makes me sick to say it, but this can be even more important than the credentialing. There are multiple stories of students dropping out to start companies and still benefitting from their college peer network in ways that wouldn't happen for someone that wasn't actually a 'true peer'.
I think this is simply a biological tendency at the end of the day and don't see any easy way of changing it.
No one does it because we intuitively understand that there is no real value in doing so; the purpose such institutions serve is credentialing, not education.
(End paraphrase)