Busy teachers had little time to develop a coursebook. It's much easier to shop around and buy a book that fits the need as close as possible even though it may leave a lot to be desired. It's like food. Often it's easier to go to the store and buy a frozen TV dinner than to buy the various ingredients and cook them yourself.
So what can the busy teacher do? Beginning teachers may have no recourse to published coursebooks. However, teachers or schools with at least five years of experience should be able to do this quite easily. There are two approaches to coursebook writing. The bottom-up and the top-down.
The bottom-up approach would be to closely study the students to discover exactly what kind of English they need making no assumptions concerning grammar and vocabulary and subject matter. However, some people would consider this reinventing the wheel.
The top-down approach would be to refer to your favorite collection of coursebooks and teaching materials and take the things you like and adapt them. If you like how the business English book used some interesting business news in each unit to introduce new vocabulary with some speaking exercises built around them then adapt this idea for your medical English students and find some interesting articles on medical news to build your exercises around.
Perhaps another coursebook presents business writing in an interesting way and you can get ideas from that to do the writing component of your book for sports coaches. Do you like the way grammar exercises are presented? You can do the same but adapt them for the needs of your particular students. You can use a four skills book written for teenagers to get ideas for oral English lessons for engineers.
You cannot copy the books but you can model your coursebook after them by taking the best teaching ideas from each one and customizing them to fit your students.
Why go through all of this trouble? (1) After you finish your own coursebook you will have something which fits you and your teaching philosophy and your students and their needs. (2) If you discover something doesn't fit or work well, almost like action research on steroids, you can easily modify it between classes. (3) You can easily update it and keep it current with the latest world developments (no more books that talk about sending faxes but not Emails) and developments in ELT methodology (corpus based research?).