I think people get scared when it comes to things they don't understand
do you not also find some people are the opposite and suddenly become experts? Which is so annoying because they know all the cures.
Sadly, in my own experiences, the % of the population who DO have a mental health illness is vastly larger than the quoted figures, they are just far better at ignoring it, covering it up or hiding it.
My own mother, narcistic, never been diagnosed yet managed to turn all the local authorities against me for MY mental health to protect HER reputation!
A homeless man I felt pity for and took into my own home while he got his door man licence back and then he assaulted me, threatened me, stalked me, conspired with others against me. Never once been treated for his clearly dangerous mental health issues.
And I could go on and on....