Look a little closer
Sunlight is refracted and reflected inside water droplets, reaching an observer at a particular angle. The droplets satisfying that geometry form a cone of light whose cross-section is a circle.
From the ground, the horizon blocks the lower portion, so we usually see only an arc. From a high viewpoint or an aircraft, conditions can reveal much more of the circle.
A rainbow is not a fixed object at a distant location. It depends on the observer, the Sun, and a particular set of droplets—which is why its end can never be reached.
