@Cameron Washington Because that is not in any way the same thing. Look, see it this way, if a caucasian person would recieve a heart transplant from an african person, would said caucasian person suddenly become a hybrid? No, they're still completely caucasian.
Similar thing here. Aizen did not become a Soul-Zanpakuto hybrid (that's not even a thing). His zanpakuto just became a part of is body, like an extra appendage.