The default mode that untrained people enter when thinking about mental illness is denial, as in, "thank <deity> that will never happen to me". Appallingly, that is ingrained in AI product safety; why would we sacrifice double-digit effectiveness/performance/whatever to prevent negative interactions with the single-digit population who are susceptible to mental illness in the first place?

We just aren't comfortable with the idea that all of us are fragile, and when we think we could endure a situation that would induce self-harm in others, we are likely wrong.