1. How sure are you that it can't? Have you tried with ChatGPT-5-Pro?

2. It's probably not very well represented in the training data. Humans don't often ask other humans to count to a million. Asking why an AI can't count to a million seems similar to asking a human why they can't easily recite the alphabet backwards (unless they've tried it before, or they really think it through ie count forwards to get each prior letter, or they use a 'tool' e.g. writing it down).