Mostly because we know very little about mental illness, so we are just guessing.
Previously, we had a terrible habit of identifying "mental illness" by external factors, such as meeting social and moral norms (so being gay was considered a mental illness, as was being troublesome).
To avoid this, and given that our understanding of the fundamentals has barely improved, if at all, we have chosen to take an approach that is much less likely to cause extraordinary distress to people who are perfectly well.
Remember that being labeled mentally ill can durectly rob you of your most basic human rights, so this is not a diagnostic that should be easily applied.
Previously, we had a terrible habit of identifying "mental illness" by external factors, such as meeting social and moral norms (so being gay was considered a mental illness, as was being troublesome).
To avoid this, and given that our understanding of the fundamentals has barely improved, if at all, we have chosen to take an approach that is much less likely to cause extraordinary distress to people who are perfectly well.
Remember that being labeled mentally ill can durectly rob you of your most basic human rights, so this is not a diagnostic that should be easily applied.