Because when you ask it to solve a math problem it isn’t actually solving the problem like you and I would. It’s taking your input and comparing it to the data that it has previously been trained on to give the most likely response. If it was actually properly solving the problem, then it wouldn’t mess up on simple stuff. You can see this if you just give it a math problem that isn’t commonly completed, like a few multiplications of large numbers in a row. It will just spit out some random large number that isn’t even close.