I have over 500 online accounts. Imagine if all of them used a login method where I had to have backup devices registered, instead of just me backing up the credentials (like I do today with a password manager).

With backup devices, whenever I upgrade or replace a device, I need to go to each of the 500+ online accounts and register the new device. This is much more work than a quick login to each site via my password manager (which can happen on-demand, only as I need to use the services).