I would suggest that in most cases if it is not tested it is not taught.
Very true.
The fundamental problem is that there is no standard trainer with standard equipment, the military can teach all their pilots the same but the GA World has to make do with a huge range of different equipment some installed and some handheld and of course there is no requirement to have any specific equipment at all.
To produce a syllabus to cater for so many variables is difficult if not impossible. Changing a syllabus is not a panacea either as most instructors teach what they were taught to teach and it might take a generation to make an effective change.
The UK CAA introduced a new PPL Syllabus in 2015 in CAP1298; how many have actually seen it or use it and is it any different to what has gone before?
The only common point is traditional Nav which remains in the syllabus. Teaching all the other variables that now exist means it can't realistically be done in the time normally allocated.