How do you know? Have you directly asked them if they have any mental health issues? And for that matter, have you asked them if they have any physical health issues too? And would you trust their answers anyway? It just happens that the FAA offers ways to get back to medical certification after having, say, a heart attack. But if you don’t trust pilots with mental health issues who are receiving treatment, why would you trust a pilot who had a heart attack and received treatment to be medically qualified? Clearly they have a history if potentially-incapacitating health problems.
As far as “glass ceilings” go (which is off topic, but since you brought it up), look into the Mercury 13 group of would-be female astronauts. The medical professionals who tested them said in many cases they would actually make better astronauts than men, and yet they never flew in space. It certainly wasn’t due to their lack of mental aptitude either.