I wonder why noone else does this its simple for new growers and saves the massive headaches that you get with nutrients. And has great results.
To answer your question, I think it's fear. Fear of failure by new growers is what drives a by-the-book approach with zero point of reference, and fear on the part of growbook authors who are uncomfortable asserting something different. This, assuming the author actually grows. Not likely.
In all truth, I think that most growbooks are a part of some pseudo-conspiracy that's designed to prevent regular folks from having successful grows. How else can one explain all this bullshit? Go down to IC, a much smaller forum populated by a good number of professionals, and you'll see similarly simple ways of controlling fertilization.
The harder you make this, the greater your chances of failure.
Simon