It's also the structure that a formal class affords that makes it more effective. This is part of the appeal of book clubs (yes, part of it is, for some people, the opportunity to hang out with people and drink wine and eat cheese): some externally imposed structure makes it easier to commit to learning something. Paying for an opportunity also tends to make people more likely to honor their commitment.

I find it's sometimes hard to figure out a quality source of knowledge, and sometimes it's hard to figure out what specifically you need to learn sequentially.

For example, when I was learning how to code years back, I literally had no idea what language I should start with. I learned from a top->down approach, but the benefits from a university is they lay it out so you get a bottom->up understanding.