It doesn't.

Before any sort of experimental drug proceeds to clinical trials, it needs to have robust preclinical data showing A) Safety in animal models and B) some functional data suggesting it would have efficacy in humans. Primary research into neurological conditions and mental pathologies is limited by model systems and the measurements we can take.

The most detailed measurements in neuroscience are terminal, invasive procedures. For obvious reasons, we do not do these in humans. So we have to use a animal model - typically a rodent of some sort. But there are enormous species to species differences that make assessing model systems very difficult.

Consider the primary symptoms of schizophrenia in a human; psychosis, delusions, apathy and others. How do you assess if a mouse or rat is actually experiencing auditory hallucinations? By what criteria can you examine a mouse model, and determine if it is undergoing psychosis? If my hypothesis is that psychosis is caused by X deviation in Y protein, I first need to have an accurate animal model for psychosis. Or I could look at the animal first, and then look at humans after, but that has it's own challenges.

The biggest limits on research in mental pathologies is developing accurate mechanistic causes for conditions such as schizophrenia. Once you have a clear idea on what the problem is - protein expression, inappropriate excitability, etc - you can far more accurately develop new therapeutic drugs, or repurpose old drugs.

It has little to do with emotional bias in research, and far more to do with technical limitations in what data we can actually gather.