Thanks for taking the time to pen your very sensible outlook and for such a thoughtful response.
I think the reasons vary per case, but denial is partly in play. Every parent wants their child to succeed, but success looks very different for everyone, and some parents just don't quite grasp that their child's success is going to look different from their own.
A few years ago I was tasked with teaching college-level business document editing to a child who couldn't read or write his own name. Parents were dead set he was college bound. We could have taught him to dress himself, which he couldn't do, or how to practice decent hygiene, or how to make friends and be a good citizen, but no. We taught a boy who couldn't dress or bathe himself or act appropriately around others how to edit documents he couldn't read because his parents were too proud of what they thought he could be to see what he was- a sweet, happy boy who loved sports and needed some serious help taking care of himself. Hell, we could have at least taught him to read.
This isn't an isolated case. We get a generous handful of kids each year who genuinely don't belong in our program, and it's not because they aren't "smart enough"- It's because the things they need help with the most aren't the things my program teaches, while the one that does teach those things is right next door.
In a sense, denial is more like a common symptom of many different problems. Sometimes it's cultural- we have students come in from overseas who were in the general education population because there wasn't even a concept of a SpEd program in their home country. Other times, it's guilt- parents thinking they owe their child all the advantages other kids get so they can't waste time on learning how to use the bathroom properly and how to not hit people. Other times still, it's plain contempt, with parents insisting that no child of theirs is going to be "lumped in with the retards."
Whatever the reason, it's one of the worst things you can do for your child, denying them the help they need the most because you expect "better" of them instead of just loving who they are.
--^@


