For the same reason that born-again Christians die. Not because they are spiritually dead, but because they are in a world subjected to futility (Romand 8). Jesus said, though you die, you will live.
Physical death is not an indicator of spiritual condition for anyone - adult or infant.
Because all men are born in sin and cannot save themselves, I believe that God applies or imputes the perfect righteousness of Christ to all those whom He wishes to save. This is what makes Christ our Savior. It is God who saves us.
If infants are not spiritually dead or unaccountable for their sins, when does God impute their sins to their account? If they are unaccountable for their sins, then what is the basis of their justification? How can an infant be saved apart from the righteousness of Christ, who is the way, the truth and the life? How does that justification satisfy the demands of God's righteousness without laying aside the demands of His holiness?
Your age of accountability theory is rather novel in the history of Christ's church. I believe that those proposing such a novel theory have the burden of supporting it. Therefore, please provide a scriptural and exegetical answer to the questions posed above and provide an argument in favor of your theory.