The problem is that the standard of theory instruction is pretty average in many areas, this being one. That, also, has to do with the syllabus, the exams, and the fact that many students are endeavouring to get through their exams with the absolute minimum level and standard of theoretical knowledge. Flight instructors beget like theory knowledge impoverished flight instructors. In many cases, the only theory instruction up and coming pilots get is from those instructors so the cycle remains viciously feeding upon itself.
I have some background in theory instruction and, I have to say, there are some documents out there which claim to be texts which are, themselves, a bit average.
At least, those of us with an engineering background and who are involved in the theory side of things can pass on this sort of stuff to our students.
It is nice to have a bit of an idea what the rules are, where they are, and what might be involved. At the end of the day, it is nicer to have useful bits of take away stuff to note in the shirt pocket notebook. I can recall my first training captain in the airlines taking out his voluminous pocket notebook on day one. It had all sorts of useful stuff in it that he had laboured to acquire and he periodically pulled it out to check if he wasn't sure that his memory was spot on for any particular item.
I learnt a lot from my training captains and checkies. Also, I was fortunate that all my PPL and CPL instructors were rather experienced and knowledgeable A grade instructors who knew their stuff pretty well.
That was back in the day when political correctness didn't come into it at all and the student was expected to learn. If he didn't, the pressure ramped up appropriately to get the message across. My first PPL instructor took along a rolled up newspaper. First time mistakes were addressed patiently and gently. Do it again and the student error correction newspaper would be applied, patiently and gently, to the back of the skull. I don't recall that anyone made all that many mistakes along the way. My group all soloed in short order (4-8 hours) and PPL tests all came up pretty much on minimum times (scratching the memory here - 35 hours to unrestricted ?)
Dreadful ? Of course, by current standards. Effective ? You bet.