You also mentioned Operating Systems, which actually is something that many people have learned by creating their own (maybe not a sophisticated system that would find a real-world use, but enough to get a basic shell up).

It's estimated that about a billion people have online access. Let's assume that everyone who works with computers, is online. So, there are about a billion who work (either in their job, or in their free time) with computers, and hence, have to learn to deal with their operating system. How many people have created their own OS, no matter how trivial, as a way of learning how to use an OS? 1,000? 5,000? Let's be generous, and say 10,000 people learned to use an OS by implementing one. You might call that many people, but it's only 0.001%.

Abigail