Actually it has been tested - the safest position is laying down, or being in a reclining position, as the body can handle far more sudden G forces in a reclining position vs sitting upright - however - with everyone's seat in a reclined or flat position, getting out of the aircraft quickly and safely after the accident you survived by being in the reclining position, is infinitely more difficult for you and everyone else. The FAA decided that somewhat-reduced initial survival with everyone upright vs the chaos, injuries and death from trying to scramble over, under, around reclined seats, or being trapped in a damaged or burning aircraft, is the better option of the two.