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.