[mattkrause]

The reused lesson plans still often need to be updated, either in light of curriculum changes or just because something “didn’t work” (too hard, too easy, messes up a dependency, want to emphasize something else, etc).

As for why there aren’t off-the-shelf plans:

You might want to adapt the curriculum to the current class of students or the broader community. The College Board does distribute a syllabus for AP US History classes, but it’s deliberately sparse so that teachers can plug in people and events that are “locally valued” (their words, not mine). A class in Alaska might spend more time on Native events and statehood; one in Boston might up the emphasis on the Revolutionary War events that happened nearby; Texas is going to go crazy with the Alamo. This is true for other subjects too. A science class might spend more time on local ecosystems that they can visit. A few of my literature class read a play and then went to see a production of it; that part presumably had to change every year, based on what was being performed nearby.

The other reason is that the teachers need to review the lesson plan anyway: no one can remember a thousand hours of material! While doing so, it makes sense to “refactor” them into something that matches your own mental model of the content. Teaching off someone else’s materials feels weird and often goes a little more poorly.