Why? All of the information is available in books and on Wikipedia. Nowadays you can learn nearly anything for free with just an internet connection and some dedication. The biggest potential impediment is be a self-defeating attitude that prevents you from trying.
On the other hand, don't expect learning "on the job" to mean that you'll get paid to learn.
Because of laziness. You are right that you can learn by just investing time, and the grandparent is right that almost nobody will bother when they can play videogames with that time.
He had a hard time doing that and valued a lot direct contact with the teacher. It resonates with me because I studied engineering but love psychology too and when I learn let's say NLP from video-audio made by masters I learn way more that in the same authors books.
Books are very good when you dominate the subject.
90% of the communication is not verbal. Nuances and emphasis are very important.
It's no different from the situation before the internet, since books are cheap (compared to your time) and usually are still a better source of information on such structured subjects anyway.
In fact, when I was in college, I mostly read textbooks anyway instead of attending lectures (it's easier for me to learn this way).
This however is not the whole story. Even if you are motivated ("I'd like to learn math"), you are probably not motivated enough for some feats of cognition. If you know people who stayed home and studied math on their own for a few years, full-time, at college level (I don't), they are probably very rare.
I've learned a decent amount of mathematical stuff on my own without traditional books, and I'm pretty sure this will become more and more common over time. I'm a member of the earliest generation (b. 1985) for which this was really possible, and I just fell into it by accident, but in the coming decades I expect we'll see a lot more people who are almost entirely self-educated through the internet. I'm sure it'll still be a relatively uncommon phenomenon, but at least the people who learn best in this way now have access to the information and tools they need.
Happily the computer science articles tend to make at least a little sense. But then I already have a degree in the field.
There are people who can learn from encyclopedias, but most people need well-paced, linearly(ish) arranged material.