This is difficult to implement for technical reasons.
The quiz has to link all information about an attempt to a particular 'user' record in the database, and each user can only have one open quiz attempt at a time. All not-logged-in users share the same 'guest' user database record. Therefore, two guests could not attempt the quiz at the same time, and even if they could, it would be difficult to prevent one guest from seeing another guest's attempts.
A workaround is to create a visitor account, say with username = guest, password = visitor, for everyone to share.