I think it's down to three things:
1) Economics. Instructors are very badly paid. Most schools will not pay for groundschool, the instructor has to earn a living, and he needs to get the engine started.
2) The syllabus. Like the ATPL nonsense we have to learn, the stuff a PPL has to learn is largely irrevelent. I always felt it was much more important to teach my students some good inflight decision making than the finer points of flapback/specific gravity calculations/maritime air masses.
3) The students. Most PPL students are 'cash-rich-time-poor' types. They treat their helicopter lessons like a round of golf. They turn up on the hour for their 1 hour of helicopter flying, then they shoot off to their meeting/home/office, and forget about flying (and any studying you have asked them to do) until their next lesson.
I have forgotten the number of students who are only interested in flying and I have gone blue in the face trying to get them to study. Then they are ready for their PPL test and have only passed two exams. Very frustrating. But I guess it's their money.
Of course you do also get keen students who study loads (probably like the people who attend the safety evenings), but they are in the minority.
Also - most PPL students, when they get their licence, stick it in a draw and never fly again, or they fly once every 3 months in the local area. All ground knowledge has been forgotten, all that remains are the basic (probably rusty) flying skills. No confidence to go on any trips and no confidence on the radio.