Bums on seats. Inexperienced and inadequate teachers need books because teaching with them is easy. Students want them because that's what is expected - a nice seat in the classroom, a nice teacher, and a nice book you can take home or to work to show your loved ones or boss.
And management want them because they take the line of least resistance and biggest margins. The publishers and authors of this tripe also make a living. Everyone but the learners is on a gravy train and nobody's learning English except accidentally.
However, one must be realistic. Give or sell learners a grammar and/or vocabulary book to allay the book-separation anxiety, (an anxiety of the teachers as well as the learners).