Two things are preventing me from doing that: I occasionally want to access my passwords in a browser (and I do not want to log in to iCloud on that machine), and I'd feel really bad about having my passkeys stored in an Apple service with absolutely no way of exporting them in case I ever do switch platforms. (Bitwarden at least includes passkeys in their JSON export format, as far as I know.)
What I dislike about Apple Passwords is how tightly coupled everything is.
I just tried to set it up on my Windows 10 machine with a local account, but it requires Windows Hello to be turned on, which can't be done except with a Microsoft account.
Kinda ridiculous of them to force arbitrary restrictions on us.