for example on w3scoul u can learn all before taking the next language
w3school is a documentation website, where you can find anything and everything on the language. Free Code Camp is a course, and it’s trying to give you the basics of the language, so that you are then able to manage on your own, and consult a documentation website as needed
For the same reason that when you are learning a language, they don’t try to teach you every word. That’s what dictionaries are for.