How come climbers on mount everest start in the middle of the night when climbing for the summit?

Answer 1

because it takes 10 to 12 hours to summit from the south col...that puts them at the summit around noon...that will give them some time in the daylight to take in the view and still be past the hillary step and other hazards before dark

Answer 2

So they can reach the summit and return to their camp before the next nightfall