To add to what you've said: There's so much to be done in the presentation of material, and so much of that needs to be at least somewhat tailored to classes.

To add to what you've said: There's so much to be done in the presentation of material, and so much of that needs to be at least somewhat tailored to classes. Even though I only substitute teach now (and teach writing workshops), I still do the same lesson a few times per day, and so will re-work lesson plans based on what is working with this particular group of students. I've had classes where I can be pretty relaxed, and we can have useful discussions, and I've had classes where I had to keep them busy lest they get into fistfights (no joke). Additionally, lesson plans need to be differentiated based on ability. The way I present participles to a class full of kids who stink at grammar is different from how I present participles to a group of honors kids who mostly get it. One could try to argue that "If the teachers before you had done their jobs, all of these students would be at the same level", but even that isn't true, as students are at different levels by dint of different innate ability levels.