Why is eight hours the "max possible (legally)?"

If you fly 12 hours, all 12 count against your flight and duty limitations, and if you are part of that crew, but not at the controls, when a violation occurs, you're on the hook for the same ASAP report, and for enforcement action.

The pilot in command remains the pilot in command, incidentally, of an aircraft requiring more than one crewmember, and may log it accordingly.

An existing discussion on the subject: Logging Heavy "Augmented" Crew Time