I haven't hit a deer, not even come close since they aren't a problem in my country. You are most probably right and i have seen videos of deer just jumping onto the road at the last second which causes an unavoidable accident. My viewpoint is that when you hit a creature(animal or human) at 80mph, they are most certainly dead. If you hit them at 60, they might survive but be gravely wounded. If are able to react and slow down before contact to about 30, they will be hurt but at least they have a much better chance of the survival. Somehow going at same speeds during the day and during the night seems very risky