There are also examples in the opposite direction. Heartbeat and breathing are active 24 hours a day. People can keep playing for hours. So it's not like any human activity is necessarily limited to short intervals. I guess the proper question would be: "Why is willpower (sometimes) spent so fast?"

Which I guess cannot be answered until we look into details of what constitutes "willpower". (And we may find that actually different forms of "willpower" consist of different components -- maybe even that different people use different mental activities for dealing with the same kind of problem -- so using a single word for all these meanings is confusing.)