If you could get up high enough in the sky, then you'd see that some rainbows continue below the horizon.
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.
But if the sun is very low in the sky, either just before sunset or just after sunrise, we can see a half circle.
The higher the sun is in the sky, the less we see of the rainbow.
The only way to see the full circle of a rainbow in the sky is to be above the raindrops and have the sun behind you.
You would have to look down on the drops from an airplane.
Pilots high in the sky do sometimes report seeing genuine full-circle rainbows.