In mathematics, your thinking process is directed towards finding the one correct answer. Maybe this is different at very high or theoretical levels, but for 99% of the population, you are just trying to find a way to be as good as the calculator. Your math abilities may reveal something about your attention to detail, or ability to apply concepts to real-world questions, but for any given set of input, there's really only one correct output.

Sometimes we use writing in the same way, but only in the most basic of essay questions that are more like a long version of fill in the blank. You store up the info, and regurgitate as needed. But, once you're out of middle school and generating a few pages at a time, writing becomes a much better reflection of how you think and understand things, not just a reflection of what's on your mental hard drive. You have to form coherent thoughts in your mind before they can then be transmitted via pen or keyboard. There's not just one correct answer anymore.