A key for each service for each device is too impractical in real life.

Getting a new device would mean logging in to hundreds of services to link up the new device. Or somehow keep track of which services have keys with which devices. And signing up to a new service would mean having to remember to generate keys for a a handfull of devices, some of which might not be available at the time (like a desktop computer at home when you are out). Or you risk getting logged out if you loose the one device that had a key for that particular service.

I agree passkeys can make sense with something like BitWarden or KeyPassX. Something that is FOSS, and is OS and device agnostic, and let’s you sync keys across devices. And should have independent backups too. Sync is not backup.