Because we don't stay dead in Christ.
Romans 6:8
Now if we be dead with Christ, we believe that we shall also live with him:
9Knowing that Christ being raised from the dead dieth no more; death hath no more dominion over him.
I believe WE do stay dead in Christ. If Christ is in me then what was me is no longer. My mind was carnal and hated God. So if we are dead that means that my mind is dead that hated God. What is new in us is the new man. Christ enters in and it is His Life that is alive in us.
Paul
1What shall we say then? Shall we continue in sin, that grace may abound?
2God forbid. How shall we, that are dead to sin, live any longer therein?
3Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death?
4Therefore we are buried with him by baptism into death:
that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. 5For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection:
6Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
7For he that is dead is freed from sin.
8Now if we be dead with Christ, we believe that we shall also live with him:
9Knowing that Christ being raised from the dead dieth no more; death hath no more dominion over him.
--Rom 6