What happens when we focus on child ‘mental illness’ and not on the whole family’s lived experience?

"3 children in every classroom will suffer from a diagnosable mental illness", why do they not say that 3 children in every classroom may be living with loss, family breakdown, parental mental health difficulties (often caused by parent’s childhood trauma), bullying, poverty, abuse, redundancy or anything which may cause distress within a family? After all, there can be recovery in all these issues.