This statement is given as a self-evident truth. In order for a will or testament to come into effect, the one making the will, the testator, must have died.
Since God prepared an inheritance for us which incorruptible (1 Peter 1:4), why should be distracted by the things of this world which are passing away. What is reserved for us, if we wait for it, is something that is so much greater, so much more enduring than anything that belong to the here and now.