From my experience college is not so much about actual knowledge acquired but mostly about learning to learn and think. I could have studied civil engineering instead of CS and be exactly as good software developer as I am, maybe even better. Is college absolutely necessary to perform some jobs? Probably not. But I think I'd benefit if my hairdresser or tailor (I don't actually use services of any of those) attended…
That's totally confirmation bias. Smart people are more likely to go to college. Almost everyone becomes wiser between the ages of 18 to 22. Why are you so sure that college is causative here? I don't doubt that college is where many people grow up more intellectually, but I have yet to see proof that it is the cause of that growth.
I can agree that it's not the sole cause. But I really have trouble believing that being exercised and lectured about the stuff you knew nothing about every working day and being strongly challenged at least semiannually has no effect on people.