Comment by kardos

1) virus database gets updated, what was written virus-free with the previous database may not be virus-free on the current database.

2) removable storage devices

3) the system drive is not controlled during reboots

You could imagine building a system that tracks which files we wrote and with which virus database version, which resets things to be scanned across reboots and virus database updates, and has exceptions for removable devices and so on, but it screams "attack surface"...