It's because psychology is speculative neuroscience. And speculation isn't science, it's bullshitting.
As software engineers on HN, we don't debug a computer program by talking about why the program might be feeling bad because it sexualized its mother as an infant or some other insane nonsense. I suspect once neuroscience gets better, and we can simply load up all our neurons into a computer and run the equivalent of GDB on it, it'll be really obvious that humans are 100% deterministic, people with psych problems are deficient in X, Y, or Z chemical or the wiring was screwed up in some place or another, and we'll be able to fix it.
Eventually we will look back on Psychologists the same as doctors that did Bloodletting and "Balancing the Humors"- complete crackpots that set science back a few hundred years.