Not all parts of Earth are covered in rain forests. There are deserts too. This is from a weather effect known as Hadley Cells. It is a direct result of the sun sitting directly above the Earth's equator and since that strip gets most of the sunlight, the air is warmer there.

Denser air rushes towards the equator from the northern and southern hemispheres and rising air produces precipitation that creates rain forests near the equator. It dries out and it heads north and south.