* Many people talk about forgetting things as soon as the test is over, but why would they remember it even that long without a test?
* A formal course is more than just reading the textbook; it's a guided tour by someone who is a professional teacher as well as someone working in the field. (Yes, some are bad. Most are at least middling and some a very good.)
* And then there is the whole issue of things you wouldn't study on your own if you weren't required to. A CS degree, for example, is more than a bunch of CS courses.
>but why would they remember it even that long without a test? If you learn something that bores you, sure. But I claim that when you learn not for a test you would properly inspect things and their domains and try to get insights into them, their necessity to the object being studied and so on.