Why is mental health not taken seriously I think there is some desire to separate the condition of mental illness from other physical illness because of how we come to understand agency for ourselves and others. The distinction may be misguided, but it does offer some sense of security for the individuals who consider themselves mentally healthy.
Having experienced infection or other physical ailments, we all reasonably accept that some conditions arise due to no fault of will for the person suffering. Recognizing the role of chance in physical infirmity, we allow ourselves to sympathize with another person who is also suffering, while making no critique of ourselves as persons or actors.
With mental illness, however, the sufferers thoughts and will come into play. Any characterization of the other's problems also reflects on our own thought patterns and will. An individual cannot sympathize with a mentally ill person, without calling into question the very process by which we come to think about reason or experience. Sympathy at a distance is less tenable. A problem with mental function becomes associated with a problem of agency. Where the mentally ill are seen as thinking wrong or feeling wrong, the relationship to an outsider requires some disconnect from thinking. "I can see how your situation involves wrong thinking, but I cannot sympathize with that kiind of wrong thinking without bringing my own agency into question."
So mental illness becomes a problem for them, not us. While the disassociation is taking place, the cause of the mental illness becomes associated with agency. In some way we come to think of the mentally ill as having brought the problem upon themselves. Since mental illness gets identified as some defect in will, the consequences of the illness are treated as naturally emerging from choices the sick person has made. To some degree we believe they deserve it.
Mental illness is not taken as seriously as other health problems, because it is perceived as a problem separate from ourselves and which cannot affect a "rational" person. It isn't treated as real as breaking a bone or a cancer diagnosis. So mental illness gets sidelined.