I respectfully must disagree, all could be trained but few are. I feel that most of the blame lies with the agencies. In the old days we taught Buddy-Breathing, we taught it and drilled it to where new divers were perfectly comfortable with it and were perfectly comfortable to go out and practice it.
There is nothing in any agency guideline that prevents that at all even now. The main issue is the divers themselves. How many of these practice their skills after finishing the course to keep them fresh? Ive come across people that may have done 20+ mask clears as part of a course but then haven't bothered trying another one for 4 years since. Agencies (or instructors) sadly can't force people to keep updated in their skills.
Lastly I blame the student for not seeking out qulified and competent instruction in the first place,
Entry level a student knows nothing. They dont know enough to be aware there are different agencies yet alone that relative merits and problems with their individual syllabus. At a higher level yes they should know enough to make an informed decision but at entry level that's not possible.
Even the best training on the planet though can't compel a student to keep practising skills long after qualification. Thats down to the individual entirely.