Finding materials that explore the right vocabulary and grammar is very hard.
Exactly. They're also something to have at home rather than just in class, to revisit, to reflect on etc. plus they all have workbooks, many have interactive online self-study materials tied to the scheme of work, videos in sequence and the newer ones have QR codes embedded.
Since different people learn in different ways it's important to go at it from different angles, and that is bloody hard if you have to make every lesson from scratch.
Plus, any given class might have doctors in who are very good at learning quickly together with people who left school at 16 with no qualifications and everything in between. And people with plenty of time on their hands as well as single parents with hard jobs.
Coursebooks have good graphics, a logical progression of learning and a wide variety of topics and type of activity. The material is also tied to the outcome of the course. Tried and tested over decades and have to be fit for purpose or they just don't sell.