We get new course books every other year or so, curricula are generally reformed every time the government changes, plus there's a lot of project work, interdisciplinary work to keep the lessons engaging and interesting for all of those involved.

Apart from that: if you just keep recycling old crap rather than tailoring your lessons to the respective students and classes, you're indeed shit (to use your language). But you'd probably do an equally shitty job no matter what you do in life. If your aim in life is to get through with the least amount of work, teaching will do just fine. There are lazy people in every sector and, unfortunately, you get away with it more often than not.

The sentence "assuming the teacher actually knows .... " is a standard sentence routinely used by the right-wing anti-education camp.