It's because ChatGPT doesn't solve problems deterministically the way a programmed computer does, it solves them probabilistically. When we say ChatGPT can't do this or ChatGPT can't do that what we're doing is anthropomorphizing flaws onto the system, derived from our own experiences of solving problems successfully. Our intuitions may view this as a flaw, but OpenAI loading the whole internet into ChatGPT and creating a simple model for how to think rather than directly programming the machine is the reason this computer is incredibly useful in new and previously unexplored ways.