And even if they are tuned into autodidactism, self-study is not really an outcome in and of itself. So I pick some topic and topics to self-study for a year while staying with my parents. Maybe I get some part-time job. Maybe I even get fairly deep into some generally useful topic. Now what when I enter university the following year?

There are probably some core subjects I could usefully have studied. But, in general, I'm probably not going to place out of classes at many schools even if I've deliberately studied for AP tests. So it's not obvious that year of self-study necessarily leads to a shortened or improved degree program.

That's not a reason not to do it of course. But self-study doesn't necessarily lead to greater personal enrichment than volunteering or hiking the Appalachian Trail does.