We can explain it with ingame mechanics and "lore". The parasite only leaves the body once the body is beyond repair. In 5E revivify only works within 1 minute of a characters death. But the parasite itself is what allows us to use exceed that limit. And we see that with companions, if they are dead dead and you remove the parsite from them, you can't revive them anymore. Not with a scrolls of revivify nor Withers. It's not perfect because technically the parasite could keep the body revivable for decades but your character also wouldn't age.

Only with True Ressurection or a Wish Spell things would get more difficult to find mechanical/lore reasons why this method doesn't work. But even there you could just say that the parasite has become a part of the person and would simply be resurrected as well.