we don't understand the brain very well, psych is somewhere between leeches and luminiferous aether.
if it was more well understood then people won't need to go to 15 fucking different therapists before finding one that helps (if you're lucky), antidepressants would do better than batters do at baseball, you wouldn't need to try dozens of different medications to find one that works (if you're lucky), and they'd take effect more quickly.