Because part of the nature is mortality. We die physically from either natural causes or accidents, etc. That is the nature of our bodies and phycical creation which God cursed in punishment of Adam's sin. This still doesn't mean the baby is "held accountable" for anything, as if he "knew to do right, and didn't" (James 4:17).The sin nature ensures that we sin and the death results from that. But even babies die because of Adam's sin.
So I take it you're saying that loss of assurance does not necessarily mean you've fallen away. OK, but still, others say if there is such a loss of obedience in the first place (leading to the loss of assurance), then you've already fallen away, and Calvin implied this was from being given by God a "vain faith".IN no way, because obedience does not bring salvation, it demonstrates the reality of salvation. YOu have confused salvation with assurance of salvation.