Your body is decaying now and replicating cells constantly the reason you "decay" after death is bc your cells are nolonger replicating and aging occurs bc your cells slowly forget how to replicate properly so in turn if you do not decay your cells would not die there for you would not age since your cells would not need to replicate and forget how to do so

Immortal, definition :Living for ever; never dying or decaying

cited from Oxford languages which is the dictionary currently used for Google

Undying, definition : lasting forever

Also the term immortal was often used for God's in mythology and gods could in fact die, but for example most Greek and Roman gods do not age

Well immortal does mean unaging and un ending so I think they were trying to find whatever word would mean undying and they ran with that