Honestly, I mostly replace parts based on symptoms. And I am right probably 80% of the time with my first diagnosis. But I run into less obvious problems and spend a lot of time trying to learn how to accurately diagnose problems. Being able to read data from the computer and understand what it means, what it should be showing vs what is happening, etc., is a real skill.

But I also understand why shops aren't good at it. They make money doing the easy stuff, really. A new alternator. A new fuel pump. Plugs, coils, etc. And, like me, they know that they can get simple issues fixed 80% of the time with replacing the part most obviously associated with the problem. And they get most of the remaining 20% on the second "guess." (And charge for all of it, of course).

So, I think it's both understandable and unfortunate. They want to stay in the profitable work, and hard-diagnosis ain't it.