Because if you can become to strong from using what the game offers and thus the game becomes to easy, people complain about the lack in difficulty. Its already the case right now. People using perfect combintation of attributs, skills, items and item mods to increase their chars damage output to levels that no npc head can take and onehit everything even on the hardest difficulty, and then they complain the game be to easy.
So if you are not such a master builder and dont go the perfect path in developing your charackter, the game stays challenging. Now with the requested feature here, it would become even easier to create an imba char of death and even more people would complain about how easy the game became even without cheating.
Saying that if people use mods or savegame editors and the like, then they are active cheating and cant complain if that action breaks the game balance but if the game allows something similar without third person editors and mods, then they will.
What you just described there is a mere illusion of keeping the game "challenging". Whether they add the ability to do a full respec or not is not going to make the game any easier or harder at all. It's merely a QOL to save time and not have people do different playthroughs over and over and over again just to reallocate a few points. This is not an ARPG. It's a heavy story focused game. Not everyone wants to sit through hours and hours of dialogue and cutscenes again for the 10th time just to experiment and optimize builds. It's not fun at all and doesn't add anything to the game.
If I want to remake another character and do a different playthrough, it should be for something meaningful - trying a new story path, exploring different dialogue options, going for a different ending, etc, NOT because I'm forced to do so due to curiosity and experimentation with builds. If a game has to rely on permanently locking my stat choices to make the game difficult artificially, then it is a cheap and lazy game mechanic imo.
Now, with that said, I'm not requesting for an option to freely swap around stats with no penalty. There are plenty of ways to integrate a full respec and still have it balanced and have every decision matter. One simple solution would be to add a hefty cost to it (say...$200-300k or something). That would be enough to stop people from abusing it and still have a door open for those who are determined to fix/change their character without a complete unnecessary restart.