Because it is. One more dialogue where someone says they can't fix her won't change that. We already knowDammon can't fix that, Gortash doesn't even try to save himself with a deal and Steel Watcher says that her engine is an old model that should be scrapped (and correct me if i'm wrong, scrapping the engine out of Karlach would kill her). When it comes to Mizora or Raphael, dealing with them would be like stabbing Karlach in the back considering she's close to mental breakdown when she enters House of Hope. Hell, that's why I think that dragging her back to Avernus shouldn't even be an option. It's a great tragedy current endings already dimnish it, I can't imagine taking it even further.
If we could talk to the Gondians and try something else, or have the option to strike a deal with someone that's offering, then even if those options fail, at least we truly did all we could for Karlach.
Talking to an apprentice twice and then calling it quits is not doing "all we could" by any stretch of the imagination.
We already know that her engine is an unstable prototype and she needs the newer version that the Steel Watch has. We don't know if replacing her engine would kill her -- because we're never allowed to even get to that point. The Gondians can make it with the enriched infernal iron that drops (which has no other purpose in-game). There couldn't be a more obvious hint that this was her path to a fix.
As for "dragging" her back to Avernus, nobody's dragging anyone. She made that choice along with us. Going back to Avernus means that she's ready to face her fears because she's not alone anymore. That's about as poetic as dying free on the surface.
It seems like all you want to do is railroad us even more into one particular ending you deem fit. Last I checked, this wasn't a visual novel. It's a game about choices weaving different fates, yet the only choice you would have for us was for her to die or become a squid. On the flip side, what we want is to have more options, so that more people can enjoy the story that they craft.
Edit: Also, this idea that each character has one narrative theme is 100% wrong. Lae'zel can break free from Vlaakith, or she can choose to stay loyal to her. That is an option that you have in the game. Gale can overcome all odds, or he can lust for power and pay the price for it. Astarion can overcome the trauma but he can also become the abuser himself. Wyll can break free from the pact, or he can choose to continue serving.
Similarly, Karlach can give up trying and die free on the surface or go back to Avernus to face her fears and try to win her life back. If her engine was never meant to be fixed, however, then that needs to be made clear in the story by allowing us to try and fail more than just once.
