Does anyone see a real reason for this?
This is actually more difficult than it sounds because of the way the AirTag is end-to-end encrypted.
It may be possible for the owner’s iPhone to send the decryption key to the iPhone you want to share the AirTag with, but an issue may arise when the owner stops sharing the AirTag with the other person. The AirTag would at the very least need to be within Bluetooth proximity to the owner’s iPhone to negotiate a new encryption key, which is bad because it means the previously shared user’s iPhone could decrypt the AirTag until the owner’s iPhone is physically near the AirTag for the renegotiation process. The renegotiation process may even require the AirTag to be re-paired from scratch, which is an even bigger inconvenience.