Because no one can get through life by being constantly told "poor you, your mummy's poor little baby, it's only natural to do this and that..." I am not saying they should have contradicted the notion that he's her son but they clearly, IMO, brought him up believing that he's her POOR LITTLE BABY and nothing else.
Some ten years ago, a ten year old student of my mum called in hysterics. There had been a call from abroad and as the only one who spoke some English the girl took the phone, so she was the one who learned that her father had just died - on the phone, not being sure if she understood what was being said, her mother crying and her brother screaming that she must be wrong. That's why she called my mum - her English teacher - to call them and ascertain that it was not true. It was.
Both children are now happy. successful young adults. But they weren't always excused as "poor little babies", especially the girl. And I do think what she went through isn't this much better than Harry - always Harry, never Harry and William, partly because William was quite tall at the time, IMO - went through behind that coffin.
Letting someone believe that they'll be forever forgiven because they lost their mum isn't the way to go.
