Because, as the apostle Paul explains:

“Do you not know that all of us who were baptized into Christ Jesus were baptized into his death? Therefore we were buried with him through our baptism into his death, in order that, just as Christ was raised up from the dead through the glory of the Father, we also should likewise walk in a newness of life. For if we have become united with him in the likeness of his death, we shall certainly also be united with him in the likeness of his resurrection; because we know that our old personality was impaled with him, that our sinful body might be made inactive, that we should no longer go on being slaves to sin. For he who has died has been acquitted from his sin.”​—Rom. 6:3-7.

These, then, have figuratively “died.” After they have the benefits of Christ’s sacrificial death for their sins applied to them, they are declared or ‘counted’ righteous. (Rom. 5:1, 18; 8:30) Their fleshly bodies, with their old personalities, are counted as impaled with Christ. They are then spiritually begotten by God, giving them hope of life in the spirit. (John 3:5-8) Their hope is no longer set on earthly things. Of course, they need the material necessities of life, but they are no longer going on “being slaves to sin,” doing the “works of the flesh.” They are cultivating the “fruitage of the spirit.”​—Gal. 5:19-23.

Since they realize that the old personality is counted as “dead” by God, they exert themselves vigorously in keeping it under subjection.

Since death constitutes “wages” for sin, the Bible principle is, “He who has died has been acquitted [justified] from his sin.” (Rom. 6:7, Kingdom Interlinear Translation) He dies because of the sin that is in him, but his death acquits him of the acts of sin he has committed.