Thats because psychologists dont understand or choose to ignore how chemistry influences our personalities and emotions. An extremely simple example from the same medical/health profession is the use of SSRI's to make people feel happy. The legal system recognises how chemicals influence our feelings because of the laws that exists on illegal drugs or drink driving.

The definition of rational is being informed enough to know what said chemicals will do in the short term and long term in order to make an informed decision, but then I'm reminded we dont get taught any of the above unless we specialise at a Uni, so most people cant make any sort of informed decision.