Responses
there are some isekais that have placed rules on their world on the limits of healing magic. they can heal physical injuries or even recover loss of limbs, but not 'natural' diseases/sicknesses that are irrecoverable, like terminal illnesses. this must be one of 'em.
granted, it makes almost little sense considering how immortality is basically conjured. but well, like the previous person said, he may not have known about it as it may not have existed 5000 years ago.
In this world magic can't cure illnesses? That seems a bit underpowered for how most magic systems work, but at least this way we know there's something at stake and that the MC isn't god, just because he's so powerful. It's good that they made limitations to what he can do.