I dunno, the browser could open a file picker and let me point it towards the passkey, or perhaps let me paste the key contents into a field?

It's been several months since I looked into it, so maybe things have changed since, but I don't see why the browser has to be hooked up to my password manager through a mechanism that seemingly can't be user-piloted nor user-intercepted. How does a browser API require some external third-party software to be installed and configured to work?! It's a public/private key pair at the end of the day, right? Not some novel algorithm, protocol, or whatever. Why can't I simply tell the browser "here is where my existing passkeys reside; place any newly generated ones here as well"? Or perhaps "here is the program to call and here are the arguments to supply when you need to create and/or retrieve a given passkey"?

I don't think I have claimed the standard itself is locking anyone out - let me correct that if I did. I have just not found a way to not be locked out of the feature itself in the products of big players like Google - my Google account(s) being the one service where I feel like I could actually benefit from using passkeys over passwords (not to mention one of those I have seen pushing the "replace all your passwords with passkeys!" messaging).

To be specific, my password manager is http://passwordstore.org/, ("the standard Unix password manager").