I agree with you. However, iOS has technical limitations that prevent background apps from running. In order to be an effective remote signer, you would have to open your iOS signer app to the foreground each time you want to sign a request. There might be a possibility of using a centralized push server to wake up the signer on iOS and show a notification to the user when a signing request comes in. But it seems much more limited than what Android is capable of doing.