FIDO keys are tiny e.g. yubikey 5c
Dedicated, external hardware authenticators are great for logging in to very high value accounts or password managers, but arguably they're not really viable for 99% of users and use cases on a day-to-day basis.
I don't like having to fetch my keys when I'm at home at all, which is where I do most of my authentications. I'd have to get up, walk to where I usually keep them, walk to my computer with them to authenticate, and then either walk back or forget the keys somewhere in the house and have to search for them when leaving (or detach the authenticator from the keychain and then not have it on me when I'm leaving).
That's why I don't like using USB authenticators on a regular basis.