In my opinion, this happens because we still see mental illness as weakness instead of what it is: a genuine illness, just like the diabetes, heart disease, etc. We as a society often blame the sick for the ailment, and alienate them, which makes it less likely people who need treatment will come forward.
Why do we still treat it as weakness? We can ask the same about addiction, can't we, as most addicts are being found to be self medicating for a mental illness. I wish I had an answer, but, sadly, humans have become so inhumane, I'm not sure there is one.