People who are experts in their fields typically don't need to make money from selling educational materials. They make enough money in their field of expertise, e.g. by being a researcher or professor. The reason they publish educational materials is typically to improve their public image or become a bit more famous, and they want as many people as possible to have access.
On the other hand, paid courses (think Udemy and stuff like that) are often made by people whose goal is to put in the minimum effort plus some marketing to sell a course. They are not experts in the field, and don't need to be, because marketing is more important. These courses end up being shallow low-quality fluff.
I've taught courses myself. We charge a lot but in my experience, it doesn't make money. It's just a social giving back thing. It's enough to cover costs of production, but not opportunity costs. But we charge because the people who come for the free courses often leave by lunch. Even when it's an expensive course paid in full by a company, people will skimp out because of "work" and it just wastes everyone's time.
It takes about 4 hours to produce 1 hour worth of slides and content. So the income can be worse than freelance coding, even when the hourly rate is higher.
Rule of thumb IMO is that the person teaching spends at least 80% of their time on the topic that they teach.
The unfortunate truth is that if you want to become an expert in a technical topic, you need to put in hard work beyond 1-3 hours of watching a video, despite what all the YouTube influencers may want you to believe.
That being said, valuable material shouldn't be available for free. You should have to exchange something for it because the author put in work and expertise to make it happen, and they won't be able to provide more or support it with a price tag of free. People have to have some incentive and be able to offset costs.
Algorithms often also control what you see so even lists like this are quickly corrupted. It truly is an 'Anathem' world now.