The dermatologists usually look at the skin from an external point of view. They will look at your skin condition and think of the best way to get rid of the acne, eczema, rosacea etc. and then prescribe you a serum or a pill that will fix the problem.
The main reason they don't rely on diet and inner causes of acne - usually in the gut flora, is because they don't have any backed up documentation for this. When they prescribe you some accutane etc. they have papers and research on other patients with results - whereas in patients with improvements caused by diets etc., they don't have documentation for this or analysis to follow to track the result.
So in conclusion they want to see numbers to fix your skin problem, this may sound a bit robotic, but it is the truth unfortunately.
Hope this helped you.
Regards