For instance, epilepsy is not considered a “mental health” problem even though it's something that's happening in the brain. Schizophrenia has to do with brain chemicals, but it's a mental health issue.
Most mental illnesses are considered 'brain' issues as they involve irregularities in the functioning of the brain. Hence, the medications and treatments. It has also been recognized that all these conditions also have accompanying behavioral and emotional components hence they are also mental health issues.
Epilepsy is not considered by itself a mental illness because it involves misfiring in the brain but by itself does not have a direct emotional or social behavioral effect. Having said that dealing with epilepsy can be stressful and cause damage to the brain and psyche some of which are manifest as mental illness. Many persons live successfully as epileptics w/o dealing with mental illness.