For example, for web development, there is a lot of introductory content but there is very little in-depth advanced content published by editors. Advanced content is usually self-published.

And not because people are not looking for that, but because it's not profitable for editors to do several books on the same topic at different levels of difficulty.

What they want is another introductory book on another topic, as they know that is what sells better.

An author getting royalties from advanced books would not be able to make a living, but by self-publishing this becomes much more feasible.