That's literally the reason why the whole Justice 4 Karlach movement exists. It doesn't make any sense for her to be the only doomed companion, it doesn't make sense that her relatively easy to fix condition is unfixable despite like milion ways to do so, it doesn't make sense in the setting and it doesn't make sense in general game theme. And yet here we are, with Larian adding extra content and going out of their way to justify why she can't be fixed and going to avernus is the only option