Os provided password manager
That's the neat point. You don't have to use one when you enter your normal login credentials and the average user like does not use one.
But with passkeys you are forced to use one and the average user will use the default one of their vendor.
most platforms don’t do it since it adds little value
Yeah because they obviously don't care about security (like machine compromise - that's why there is 2FA in the first place), which contradicts all advantages of passkeys.