Because in my day to day life wearing clothes is not a prerequisite for interacting with technology. I frequently exercise. This requires many showers and many changes of clothes. I walk around in the morning just wearing my boxers for the first hour of the day or the last hour of the night. This isn't exactly abnormal behavior. I always have my phone around but always having my phone plus always having my YubiKey is a step up in difficulty and not a linear one. Any successful auth solution better be prepared to solve such behavioral issues or they won't ever reach sufficient adoption levels.