Because the email server (or application server or whatever is creating the text for that email) can access it to decrypt it. If that server gets hacked, then you can decrypt all of the passwords.

As an example: The password list might consist of entries encrypted with a public key. The private key might only exist on an air gapped laptop with its communication ports filled with epoxy in a secured facility using trusted humans to enter the encrypted password and retrieve the decrypted plaintext.

I have no reason to believe the above is true, but there is also no reason to automatically believe there is a mechanism available to attackers of a compromised machine to decrypt all encoded materials on the machine.