So if you write a book about Python, you should include the full source code to Python as well, right? If you write a book about Java, the full source code to the JDK, and so on. Because if the readers have to download and install that by themselves, the book would not be self contained anymore, URLs may break, etc etc.

Ah, no, programming language runtimes are Ok, no need to include them. But then if you write a book about React, or MySQL, or... well, you get the point.