This is probably a Linux issue. Mac OS and Windows implement the FIDO2 Platform API, which allows them to act as authenticators themselves. Linux does not. See https://github.com/linux-credentials.
But that's another point, I do use many OSes so being locked in to one ecosystem is not an option. I must also have the option to back up my credentials at all times (eg a cloud service will never suffice)
But yeah I should have mentioned Linux. I thought it was the norm here really especially among people advocating against corporate ecosystems.
I use Bitwarden, I use Android, I use Firefox on Linux. Passkeys work just perfectly fine on all of the sites listed here thus far.