the real reason is that it can get leaked....the exam papers are available to teacher in the uk 24 hours after mainly because due to clashes many students may sit exam later than time everyone else sits them...and to make sure questions are not shared among teaches through diffrent social media platforms the exam papers are not given.
students never get access to them as these exam papers are then used as mock papers within school for next year or current year 12's thus they can't cheat as well