That's because when the sun and rain combine to make a rainbow, they really make a full-circle rainbow. We usually don't see the full circle because the horizon of the Earth is in the way.

To see the full circle, however, you will have to be in an airplane, literally above the clouds. Or you could create your own ...

If you were high up in a plane, or up a mountain, you would see a rainbow as a full circle. The next time it is a sunny day you can even create your own rainbow! If you use a water spray or hose ...