Because school is a place to be an individual, have fun and learn not be restricted to your learning they need o know different learning styles and explore outer sources not just stare at a tex book, also because there is only so much a textbook can teach you. And in English there are so many skills to master for example public speaking a text book can NOT teach you public speaking or how to get up in front if an audience and say a speech. I guess there are pros and cons but there are so many more cons to teach just out of a text book which as I said can only teach so much and some people might find watching videos on English easier then studying a text book. So it's so obvious that no outer sources would be boring, bad and monotonous
The textbook may serve as a good guideline, but sticking entirely to the textbook is not a wholesome idea as I think English has a very wide learning scope and can be learned in many interesting ways. Why should we restrict ourselves? Learning English would be more fun and interesting through group presentations, videos, movies, songs, games..Etc.