I've been thinking about this, and I wonder if it is a result of students wanting "instant gratification" or a "silver bullet" or a "black or white answer". Or the teachers/writers finding it easier to give the answer.
I know what you mean. If you give people a set of metrics that should always be >= x on any type of Oracle database and this paves the way for "tricks and tips" on how to make sure you are hitting x or above and how you can ensure that you continue to do so. People like these kind of books and courses (and I admit I used to be one a couple of years ago). You think that for a small fee you will know everything you need to make any database run efficiently! Each new tuning book seems to be the same as the last e.g. a collection of "hot" SQL scripts, and a cottage industry in born. This seems to be changing now though, what with Milsap and Holt's new book. I reckon I've learnt more from reading that than I have from reading most of the big Oracle Press books combined!