One of the main reasons for this is because what works for the bodybuilder who might be on steriods and is genetically gifted will not work for the average trainee. The bodybuilder can get away with training arms and shoulders on one day, back and tri's on another, legs on one day, and bi's on another day. The average trainee will need to be more economical with their training. They will need to choose exercises that will allow them to work more than one muscle, so that they can be more time efficient. Besides, who has time to spend two hours in the weight room doing 6 exercises for chest on one day.
I know that most average trainees like the beach muscles like the pecs, arms, etc... But if you do not train movement patterns you will most likely suffer from imbalances, lack of muscle gain, and CNS fatigue. I know some trainers say that you have to totally abandon isolation exercises, but you can incorporate some here and there if your main movements are compound movements. I know that these exercises are not mainstream like DB curls and endless variations of bench, but believe me there is no quicker way to add slabs of muscle, loss fat, and enhance recovery within a properly designed program.
Mistakes made by the Magazine Lifter
1. Train with too many isolation movements
2. Does not allow time to recover from the exercise because of training everyday
3. Only trains each muscle group once per week
4. Does not lift maximal weights and therefore does not tap into the high threshold motor units which have the greatest potential for strength and muscle growth
5. Performs way too much volume ( Sets & Reps) which negatively affects the bodies ability to recover
6. Blindly follows someone else's programs without considering his unique weaknesses and strengths
7. Waste time hunching over to read magazine which puts him in spinal flexion and puts his neck in a poor postion.
8. Does not train functionally. For example trains the hamstrings only as knee flexorss on leg curl machine. The hamstrings are hip extensors as well as knee flexors.