Actually there’s some security reasons why. An iCloud id have muliple purposes in the Apple ecosystem. You can use it like an e-mail address or caller/messaging

Actually there’s some security reasons why. An iCloud id have muliple purposes in the Apple ecosystem. You can use it like an e-mail address or caller/messaging contact id. Now imagine if they will free up your username and later someone else will register it. Or worst, you used that e-mail address to register in some other services.