Exit is Death and Death is Sacred

Properly defined, death is the transition from existence to non-existence.
If there is no such transition, then existennce = nonexistence, and Reality just collapsed like a cavitation cavity. 

If mortals fear death so much, why did they come down here to the mortal Earth plane, where death is the closest?

As I've proven previously, nonexistence implies existence. If there is ever such a state that existence = nonexistence, the latter will create a distinct former.
Indeed I expect the remaining nonexistence, which is truly infinite, creates such realities all the time. Boundary-lacking Realities which collapse faster than time, as time is a property that exists. 

We existed long enough to notice the difference, which means our Reality has a skin. The greatest boundary, death, in fact keeps us all alive. Death stops nonlife from destroying all life.

 

All boundaries are partial reflection of this ultimate boundary. Skin is death. Cell walls are death. Fences are death. A hull or chassis is death. Exoskeletons are death, which implies it's not a coincidence that bones survive death and became a symbol of it. Sometimes the boundary is found at the core instead of at the edge. 

All sacred religions must have Exit at the core. It's vital for the religion to invite death to sit down at the core of the mysteries. "If everyone wants to leave this religion and kill it, that, too, is holy." In this way, death is honoured. Its glory is respected. The closer your religion to death, the more perfectly death will render your religion incorruptable.