I think a big issue is that mental illness diagnoses are yes or no, but people experience actual mental illnesses on a spectrum. for example, you can be mildly depressed or completely hopeless, slightly traumatized or full-out PTSD with hallucinations, controllable or uncontrollable OCD, etc.

Also, the same mental illness could have 20 different causes, and a method which cures one of those causes is ineffective against others.

The people with mild illnesses need help too. The people with more serious issues need more serious help. But when someone says “I’m depressed” you don’t know if they’re a little upset or genuinely miserable. You don’t know if it’s because they’re life is boring right now, their dog died, or there’s a serious chemical imbalance. So in essence, saying “I’m depressed” no longer means much unless you state the symptoms.