Diagnosis requires critical thinking which is difficult to teach. It's more like something that's "conditioned" over time and multiple experiences. I had a more shotgun-parts approach as a teenager when I first got my Duster but now I'll spend days or even weeks researching a problem before I pull the trigger on buying something. Also the necessity of having to save money due to limited budget forces you to diagnose as opposed to waste money on parts.
I'm just old enough now (33) to start noticing how "times have changed". At any given point in history most of the humans in any society are stupid and can't think for themselves (but they sure believe they can!!). What changes are the skills that are considered important or essential. I'm sure back in the 1930s the old train engineers were complaining how the dumb young guys didn't know how to keep up a steam locomotive and how diesel was ruining everything because of their relative simplicity. "Same ****, different smell."