I really don't understand the negativity here. I'm no expert on teaching, but I've definitely heard that some teachers come up with lesson plans in their first couple years teaching a subject, and then just re-use it for the rest of their career (or until the standardized test changes). This seems like a huge waste of effort, because there are so many other teachers covering the same content, with largely similar lesson plans.

If customization is so important, what's wrong with applying it to a purchased lesson plan?