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 ...

Thus we see only the top half of the circle and are led to believe that we are seeing a semicircle rainbow. Full circle ...