Why Do Schools Use Crummy Books?

From npr.org

There are some bad textbooks out there. They're often in-house productions made by non-native English speakers. Not that there's anything wrong with that, but if they're going to do something they need to do it right.

• Proof-reading and fixing mistakes is one. 
• Having good content is another. 
• Not breaking copyright laws is a third.

You know the sayings, "if it ain't broke, don't fix it?" and "don't re-invent the wheel", well, there are plenty of fantastic coursebooks out there, written by professional ESL writers who often have decades of experience. While I agree that you'll have to tailor the books to suit your students' needs, you might still be able to use them and just supplement a bit rather than writing a whole course.