In 2FA, why can’t the second factor be the computer? [duplicate]
When two-factor authentication is described to me, people always say that it's important for security to demonstrate at least two of 1) something you know, 2) something you have, and 3) something you are. It seems to me that the specific computer I'm using to log into an application should qualify as "something I have," but I've never seen anybody propose that as a second factor, even though people are happy to use a "leave-it-plugged-in" Yubikey as a second factor. Why isn't this more common?
This is NOT a duplicate of the linked questions because I am asking specifically about the use of a computer as a second factor, and the linked questions are about the use of phones or keys as a second factor. Since phones and keys are regularly used as second factors, and I'm asking why computers are never used as second factors, the questions are completely different.
