But remember you're doing something novel so you've never hiked before, and so you can only estimate how much you can do each day. You may have done other travels but never walked that distance. Even if you half the speed you'll still be way off. And this is just 400 miles, what about a walk that across the whole US? Can you predict that Susie on your team will fall ill for a few week. How many meetings will management need? What about feature changes? How much time do do you have to estimate? So you have months to plan out your walk? Most projects need estimates within minutes to hours. And so on...