A philosophical question to be sure - and I don't know that soliciting a response from me furthers any kind of solution. Having spent a significant amount of time with the TALCB as both an investigator and mentor, though, my thought is that some folks were never taught these things. I can't tell you how many times I've introduced sensitivity analysis during a training session and the folks were just blown away. They'd never seen it (in practice or otherwise). Same goes for grouped sales analysis. Most have at least heard of paired sales analysis - but don't really know how to employ it. Don't even mention regression - eyes start glazing over, mouths start drooling, etc.
In answer to your question, though: in my opinion, they 'trained' to take (and pass) a test. They weren't trained on how to actually 'do' an appraisal. They weren't trained properly in the classes, because most CE schools are primarily concerned with turning a profit - not in educating appraisers. They have not forgotten - they never had the knowledge in the first place. The core issue is: the appraiser training system is broken - and has been since initiation of the appraiser credential in 1989.
I know I'm in the minority RE PAREA, but IMO - ANYTHING is an improvement to the broken POS training system we've had in place for the last 3 decades. Appraiser training should be done by professional trainers who aren't motivated by profit, but by the desire to impart useful knowledge. How do we do that? I have no idea.