Speaking from personal experience, if a 3 or 4 year old is displaying symptoms of depression, there's shit happening at home and the parents are neck-deep in denial about their role and pigs will fly before they get help for their kid.
Depression definitely can be caused by abuse. In fact if you're being abused you are probably also depressed.
Depression and other mental illnesses are also often caused by things that can't be externally influenced at all.
Casually linking poor parenting with mental illness enforces a stigma that discourages people from seeking treatment for themselves or for their children.
Imagine if the child is sick (eg in pneumonia) it wouldn't dismiss the illness if I explained how the child got sick because their parents left them outside freezing.
It's not dismissing, it's analyzing the root cause, to try and help the child. If something causes illness (mental and/or physical), we should take care of both the illness and the cause.
In the case of depression, finding a true unique cause is still open for debate/research, but it shoudn't prevent us from helping someone who's depressed.
(Disclaimer: this happened to me. I have confirmed ADHD and maybe aspergers too, but my parents were so offended when schools asked them to take me to a doctor that only now in my adulthood I am being treated properly)
"Rather, depression has many possible causes, including faulty mood regulation by the brain, genetic vulnerability, stressful life events, medications, and medical problems."[0](https://www.health.harvard.edu/mind-and-mood/what-causes-dep...)
That isn't always true. Some people are depressed without an external cause.
It's easier if there is an external cause: Fix the cause, fix the depression.
The folks doing this research know more than you do about the causes of their depression; and they know more than you do about the difficulties in addressing any causes. But unlike you, instead of implying that they're foolish for trying to make the world better, they're just trying to make the world better.
Most victories are long, hard slogs. I respect people who undertake those slogs, knowing that victory is not assured. I have no respect for those who throw rocks at them.
As for expecting parents to notice; a trained psychiatric professional cannot diagnose depression by regular life observation alone. They rely on self reporting, or suicide attempts. Anything less than that is extremely hard to pick up on.