Because LLMs don't understand anything. That's the tech. They can only predict what they have been trained with and fail daily at the most basic tasks. Granted they can do amazing things, no question there. But they are not "smart".

For example, it seems that even at Fable scale, simple concepts like the passage of time or (gasp) timezones elude them. I live in UTC+10 and with any RFC8339 data LLMs are constantly confused - is it Sunday the 10th or Sunday the 9th, etc. I have tried many solutions for this and every time it finds a way to get it wrong.