Here's why:

* The sun is on the other side of the Earth at night. The Earth rotates, and the side facing away from the sun experiences night.

* Clouds are in the Earth's atmosphere. They can block the sun during the day, but they don't move to the other side of the Earth at night.

It's important to remember that clouds are illuminated by the moon and stars at night, which is why we can sometimes see them even when it's dark. However, they are not blocking the sun.