Interesting question 007. Can't answer for all- but my impression from what I am seeing:

From the patient angle: They are admittedly addicted to the medications they are on. So changing medication fills them with all the emotions that involves- including fear. They might not be really looking clinically right now at what is 'best' for them in the long run.

From the psychologist/ psychiatrist angle: Trial and error of treatments means years of patient visits and getting paid for those visits. A head-scratcher but perhaps justifying not using science in order to continue to value (and not put in question) their professional contributions? (As many are sincerely trying to find the best outcomes for patients.)

IMO- this -and other human dynamics- are going to be directly related to how genetics testing is normalized/accepted.