Since Jesus conquered death why do Christians still die?

Thanks for your question. The Westminster Larger Catechism asks the same question. Question 85 asks, “Death being the wages of sin, why are not the righteous delivered from death, seeing all their sins are forgiven in Christ?” It answers,

The righteous shall be delivered from death itself at the last day, and even in death are delivered from the sting and curse of it; so that, although they die, yet it is out of God’s love, to free them perfectly from sin and misery, and to make them capable of further communion with Christ in glory, which they then enter upon.

Let’s examine this answer more closely. Notice the future tense ("shall be") in the first part of the answer; “The righteous shall be delivered from death itself at the last day.” This means that death still presently reigns, but one day in the future - “the last day” - it won’t reign over the righteous anymore (1 Cor.15:54). So, until Christ returns in all his holy splendor death remains a curse for our sin. Until the day of the Second Coming all of us shall return to the dust from which we came (Gen. 2:7; Job 10:9; Eccl. 12:7, et. al.).

The wages of sin is death (Rom. 3:23). The wicked will not escape the consequences of their sin. No matter how much they attempt to drive the reality of their death from their minds, within the deep recesses of their souls they still know that they will die and stand before God as a wicked sinner. No mater how they attempt spin death, dead is dead and judgment will remain eternal.

However the case of the righteous is far different. Death to them is a blessing. Physical death will end all their misery (Rev. 21:4). All things will be new, enjoyable, and everlasting. They will not be able to sin anymore. There will be no more battling the flesh or others. All will be at peace.

And it gets even better. There will full communion with God himself. They will know the fullness of God’s love even more completely. There will be nothing in us anymore that blocks the full reality of God’s glorious embrace. Nothing will be able to block us from God’s wondrous presence. What a day that will be!