Biometric keys are still a niche techie thing that the average person probably doesn't even know exist. Most people will be using passkeys exclusively through their phones, often unintentionally. And outside the first world it is not uncommon for people do own no computing devices apart from their phones.

Backup keys and recovery codes also do not solve all cases of key loss. One thing I worry about is what happens if I am traveling in a foreign country and loses my belongings. In the past if I can convince someone to let me use his computer I can at least log into my email account as long as I remember my password. If everything is passkey then I will be locked out of all my online accounts until I make it back home, assuming that I have actually properly set up the backup device and keys. Humans are not very good at making sure that backups actually work.

You can’t expect your grandma to go to those lengths. Heck, even most internet-native people probably wouldn’t.