I think OP didn't mention anywhere that he can see his wife's AirTags, also he actually mentions family sharing with obviously you need 2+ accounts.
I think he means like if they can see each other devices, why can't they see eachother items (Find My trackers) too?
Just re-read the thread, I must have been on drugs because I don't know why I replied that.
To answer the question its because of end to end encryption.
When you first setup a Find My network accessory such as an AirTag the AirTag stores a public key and the iOS device stores a private key plus a 256bit secret in Keychain. The AirTag is also registered with the Find My network, I believe the secrets hash data is also sent but dont quote me.
When the accessory is lost it broadcast its public key plus an identifier via bluetooth. The finder device receives this, adds its own location data, encrypts it using the public key and send its to iCloud. Somehow this indexed with Apples servers.
The owner can then pole the servers using the solution to the hash data found in their keychain and download the location data. Then using the private key also found in Keychain the accessories location can be decrypted locally by the Find My app.
iCloud Keychain data is only stored locally and shared across a single users devices (AppleID). Obviously Keychain passwords/keys/data aren't shared with Family Sharing members so at no point could they get access to the AirTags data nor would they be able to decrypt.
The AirTag is just a device that tells other users to tell you where they are. This so happens to be close to where the AirTag is. Its not an iCloud device and it doesn't communicate with iCloud or Apple. It doesn't know its own location. Its just a little bluetooth beacon that activates a very complex series of events for you to determine its location.