Maybe because not everybody who is good at chess is also good at math. I knew a chess prodigy who could not add two three digit numbers if his life depended on it. It's a bit like catching a ball: when looked at it analytically you are solving a whole bunch of equations in real time, but in practice it is a skill that even a dog can perform with remarkable accuracy. Obviously, chess requires some analytical skills. But it does not necessarily indicate an ability - or a desire - to do maths.