It's not a technical issue that's preventing Apple or Google from making their passkeys importable/exportable.
As you stated, if you are ok with your secrets being unencrypted, then sharing is totally possible. But this seems to walk headlong into having to trust everyone that you share the unencrypted secret with?
It is also important to note that most attacks are phishing-based, and passkeys are phishing-resistant, and will only work on the originating website.
Proton Sentinel also helps to protect account takeovers, even when an attacker has stolen your password.