Because by very nature of the fact that they were travelling in your car it makes it easy for the other party involved to claim that they are not independent.

Any witness will be considered by the court but the judge will decide if they are biased or not. Prior to going to court however most insurers will not "bother" with non-independent witnesses as the customary response from the third party will simply be that they are non-independent and therefore cannot be considered unbiased.