Originating a class can be a slog and originating two to lecture next term would be a grind. If you are not professor of record and thus likely did not originate the class then your assignments and reading, hell the entire course is likely layed out for you. Teaching a section of a course you originated or that someone else originated is likely 80% of the teaching load. Familiarizing yourself with this material so you can effectively teach it is doing your job, not grinding.
I'm not sure why you feel the need to suggest I am a crappy teacher or that I lecture from notes more than a decade old. Knowing how accreditation works, how courses are originated, and how sections of thoses courses are taught at r1 institutions, I can only conclude you are ignorant as to how these things actually work.
As a real world example, there are many great online courses from places like MIT and Stanford. Hell you can sometimes even get these online lectures for free, the same material covered by those institutions as part of their degree programs.
How much grinding does it take for a lecturer to to proctor such a class, particularly given that they also likely get FAs to do labs and grading?
If you attended or work at a college where professors grind out new classes every term then that institution is going to have an accreditation and/or tenure issue as "special topics" dont make a degree program.
Now what may happen is forgetting what it's like to be a naive, 18 year old freshman in college, and not somebody embroiled in academia for the past few decades.
But the audacity to suggest that it's even remotely within the norm for professors to not know their own material... wow.