Can someone explain why Ra says we must die in 3D to reach 4th?

[Peregrinus]

Actually, I think it's definitely within the realm of possibility that humanity finds a cure for natural death in our lifetime. I think it's worth considering.

They already have a cure for natural death. It is called unnatural death

You must be young. If you were ninety you wouldn't be thinking so romantically about eternal life in these bodies. Personally, I know we already have infinite life... what we perceive as death is as the child that perceives loss at a toy being taken away, that which occurs from a limited perspective.

I heard it aptly put once: "No one gets out of here dead."