Because a certification number is public. Before your card moves at all we read it against the grader’s own database, so we already know which card it is and what grade it holds — which is why an offer made from photographs is close to final rather than a guess.
A raw card has no such record. Its condition is an opinion until a grading company states one, and we are not willing to make our opinion binding on you, or yours binding on us, sight unseen. You can still sell a raw card to us outright; it just cannot be used as part payment.