There's also the aspect that it can be a form of Narcissistic abuse. How can somebody completely convinced of their utter superiority in all things, possibly have an officially defective child? When their genes were clearly the most perfect genes of all?
No, there can't be a reason encoded in the child's DNA, it has to be that the child is merely lazy, intentionally pretending to be stupid, clumsy, deliberately trying to show them up by choosing to be physically or mentally inadequate, that there is absolutely nothing wrong at all that it's all completely in the mind of others (for the latter, this is often interpreted as teachers picking on their child, the Machiavellian conspiracies of Big Pharma inventing something to sell more drugs or the other parent clearly having Munchausen's by Proxy)
And if the differences and difficulties cannot be dismissed any more, it's declared that would be of no use whatsoever to label a child of theirs with an explanation, diagnosis or official recognition - because it would be humiliating for the perfect Narcissist to have a child carrying that diagnosis or, if the diagnosis came unexpectedly (such as when they thought their child was being assessed for being far smarter than any mere human, only to be told that yes, she's smart, and she also has ADHD/ASD), that it must never be shared with the child or anybody else, it has to be kept a secret at all costs and denied throughout. Which is where you also see external factors being blamed for symptoms - blue sweets, fizzy drinks, sugar, vaccinations, lack of exercise, boring teachers, constipation...
After all those centuries of children being harmed by not having access to medical treatment/support because it cost money or didn't exist, it must have taken time to realise that actually, some people don't want their children to access medical treatment/support because it hurts their ego for them to need it in the first place.