If you own only one device—a mobile phone, like the majority of people in the world—and you lose it, how do you get back into your passkey-protected email account?
Also, sites can and often do support multiple passkeys, which opens up the possibility of allowing for reliable account recovery using one or more backup devices (one or more YubiKeys, for instance) that is still siginificantly more secure than reset flows using SMS or email.
In other words, my email can be signed in from anywhere on purpose. It's a feature. The chances of me forgetting my password is zero, because I can back it up 7 way to Sunday.
A passkey seems to miss these basic accessibility issues. For a random website, that's fine, but for the service all of websites use to auth me, access is more important than locking it down to one device.