I can kind of sympathize with the difficulty the Forgotten Realms setting imposes on writers, especially if you're trying to have a tragic ending. I've DMed quite a few tabletop D&D games over the years, and one thing I always do is tell the players that, in my game, magic to raise the dead is not available casually or easily. If you're bringing someone back from the dead at all, it'll be a whole quest to achieve it. I do this because the ability to raise the dead messes with the ability to tell an interesting story- it's very hard to have a meaningful threat of death in a universe where your surviving friends can just pay a cleric to bring you back in a few days. Sort of like how time travel can totally remove any stakes from a story if you handle it badly.
This is one of the major downfalls of computer gaming, the easy reset on death. There have been games with real consequences for failure but people complain the game is too hard. There are ways for players to introduce those consequences in their own personal play but would rather cheese the game mechanics and then complain about the game being to easy.
When I DMed I was like you. If a player died the party would go to try to find a major temple. At that temple the Priest would give them a task they had to complete, not require gold from them. The Priest would often allow the character to be raised but then the party had a time limit to complete the task or the player would die again, an expiration date if you will on the resurrection. I had a group once try to cheat a Temple for a resurrection. The character lived but they found that temple no longer offered them any services going forward and after a short time no temple of that God would offer them any services.
These types of consequences and nuanced play are really hard if not impossible to do properly in a computer RPG and so to keep the masses happy they introduce the easy resurrection system.
As for the specifics of Karlach. As a DM I would have allowed a way to "fix" her. However it would not be simple. A wish spell, as some have suggested, is SCARY to use and should be an act of desperation. Various other heal spells would require either a VERY high level priest and/or VERY expensive spell components and sacrifices. As with resurrections I would have required a very difficult task to be completed in order for the casting to be done. (money would not be enough) I would further add a time limit in based on Karlach's needs to add more "drama" to the questing. This is all however outside the realm of the BG3 project, would make a nice added content item
