Could it be that we humans are afraid of running out of things to do? After all, if we finish that last task, it would seem that we no longer serve a purpose. As long as we have one task left, there is still hope that we can find more to do.

Of course, that is a poor way of thinking since all it does is make you lose your sense of purpose before you actually run out of tasks, but the human brain is weird like that sometimes.