Because the corollary is that a child that is not deemed 'bright' is presumably 'dim'. Do people say 'bright' to be coy when they mean clever? Or is it shorthand for the child that is not clever in any measurable sense, but has outstanding qualities (visible only to its parents).
This rant has come about because two canny snake oil saleswomen authors are giving a talk at Ds's school this week, (or next - I won't be there. so haven't bothered to check date) to sell their book which about something like 'how to recognise and challenge the bright child' Clever stuff, of course 'cos presumably all the paretns who think their child is 'bright' (which is probably everyone) will turn up to preen about their outstanding brat prodigy and pay the cash for an autographed copy. Which leads me to think - the child may be 'bright' (cringe emoticon required)- but the parents cetainly ain't...