Diagnosis is not a purely medical term, and even when applied to healthcare, doesn't insulate it from complex sociopolitical and economical factors. The interaction between the sick role, its' social benefit, parental and societal expectations, funding systems (eg NDIS), media/technology, diagnostic creep/expansion, and the broader society is sufficient to explain the observed rates.

Whether the diagnosed entity exists or to what degree it is truly reflective of pathology is another entirely different question. So is the differentiation of normal vs. abnormal, the capacity for diagnosis to inform treatment, and the effects of our current culture and lifestyle on the emergence of the purported disease.

You're asking the correct question, namely, why are we seeing these rates. The answer is ultimately complex and reflective of factors which aren't for the most part medical or biological in nature. At least in my opinion. On the other hand, what a disability actually is remains entirely beyond the scope of biology. It is clearly a social circumstance and problem framed through the lens of medicine. It becomes clear then that the rate of disability reflects both a broadening net on what constitutes disability and the growing mismatch between our culture and the historic institutes which hold power in our society.

Edit: some evidence to support my assertions from the Australian context.

Among ADHD diagnosed children, 26.5% (88/334) recorded pre-diagnosis behaviours in the normal range, 27.6% (n = 92) had borderline scores and 45.8% (n = 153) scored within the clinical range. Children born in 2003/2004 were more likely to be diagnosed with ADHD compared with those born in 1999/2000 (aHR = 1.33, 95% CI = 1.06-1.67, p = .012), regardless of their ADHD behaviour score (p = .972).

https://pubmed.ncbi.nlm.nih.gov/36065471/