It’s just my opinion, but I suspect the reason stems from this statement in the document about deleting your AppleID (How to delete your Apple ID account - Apple Support):
“When you delete your Apple ID, Apple makes our best effort to delete all personal data associated with your account. However, we are required to retain past transaction information for financial reporting purposes, and we may be required to retain certain information to comply with a court settlement or other ongoing legal procedure.”
So while Apple deletes almost everything, they are required to retain some data tied to your deleted AppleID. Since an AppleID is just the email used as the login ID, they cannot allow a duplicate and active version of what would be the same AppleID as the deleted one.