Security Keys are often used as a second factor, but the original vision was that they could also be a primary factor in lieu of a password. Humans are bad at passwords, but they carry around house keys every day. Why not also carry around computer keys on their keychain?

For the average person (not worried about a megacorp or nation state attacking them), TouchID is also a possible "password replacement" for the primary authentication factor, although it comes with the disadvantage of not being rotatable.