That's because Wikipedia isn't a tool to learn mathematics. Wikipedia articles (on anything) are not meant to teach about a subject (in the sense of schooling)

That's because Wikipedia isn't a tool to learn mathematics. Wikipedia articles (on anything) are not meant to teach about a subject (in the sense of schooling) but rather to give an exposition of a subject. It is considered to be very bad style on Wikipedia to write articles that try to teach the reader as a textbook or a class would.