Because they almost expected him to win, purely because he is bright and knows every nut and bolt on a Yawn-200 Jet. They came across as not very nice people as the series went on. At first I throught he was sweet, then realised he acted in a rather arrogant nature (which I am sure is either copied from or not corrected by his parents).
He will never be able to work for anyone, nor do I think he has the smarts to invent a new jet engine or train. He can remember everything about what he finds interesting, but that won't help him pass and english, geography, history, etc exam will it?
The parents need to have a loing hard look at him now, project what that will mean for him as an adult, pick themselves up off the floor and start working with some bloody good child/educational psychologists to get him more 'rounded'. Can you imagine working or living with him as an adult?