They are affected but because they also use and generate magnetism. On the top, bottom, and both sides of the tube you will find a large coil of wire. This is magnetized to bend the streams of electrons from the rear of the tube to diffrent point on the front. Your screen is painted one pixel at a time because you only have one tube and one electron stream generated by your 3 electron guns (green, red, and blue). The coils bend the stream back and forth and up and down the screen extremly fast to refresh the screen. When you get magnets too close to the tube it interfers with the coils and doesn't allow them to bend the stream properly. Maybe a little to much info but I hope it helps.