The training is one part but its also how they work on a technical level.

They're trained to always give an answer and never give responses like "I don't know" and "I didn't understand". They could be trained to give these types of responses more frequently, but I'm not sure it fixes anything because of the technical aspect.

Technically, LLMs have no understanding of words, let alone sentence structure, because they're mathematical models. Input words are tokenized (turned into numbers, essentially), run through a large equation that was crafted and tuned during training, and the resulting output tokens are turned back into words. They can't be confused by ambiguous sentence structure because they don't parse sentence structure. They can only ever be trained to appear like they don't understand in the same way they are trained to appear like they do.