Why is it considered foolish to postpone spiritual efforts for a future life?

Chapter 8 explicitly calls the idea of postponing spiritual practice to a next life 'foolish.' The reasoning is that the human body is an incredibly rare 'prize' obtained 'by great good fortune and gathering millions of merits.' It is not something that can be easily re-attained. The text warns that there is 'no certainty that this same form will return.' Based on their karma, some sinners may return to the womb, while others who are 'even worse' could regress from the animal kingdom back to a 'stationary (plants/stones) state.' Therefore, wasting the current, precious opportunity of a human birth is a grave error, as the chance for spiritual liberation might not come again.