We aren't as ... ummm .... "accepting" of mental illness symptoms like visual or auditory hallucinations as an indicator of someone being a prophet or a seer than they were in ancient times.
We like our 'whackos' to be on psychotropic meds, ostensibly to help them become functional and productive members of society; however given that sometimes meds create other problems while "fixing" others, I sometimes wonder whether we medicate these folks because it helps us feel more comfortable around them.
People have been using plant-based medicinals for thousands of years. The human race hasn't become extinct because of it.
My take on why the WTS is so restrictive is that they prefer to keep their followers in any kind of pain and as much pain as possible, be it physical, emotional, mental, etc., because they want to be the sole purveyors of hope and happiness. If you can get relief from medicine - regardless of its source - their hold over you is threatened.