Repair mechanisms cost energy even when they're not being used. Immortality requires much better repair mechanisms, that makes the owner less competitive. Thu

Repair mechanisms cost energy even when they're not being used. Immortality requires much better repair mechanisms, that makes the owner less competitive. Thus the immortal loses the evolutionary race against the mortal. In practice immortality in a complex organism requires many steps, those will never be done because they up the energy budget and put the creature at a disadvantage. The fact that it's an evolutionary disadvantage not to be replaced with superior offspring is irrelevant, that implies intent which evolution does not have.