Why is this the answer?

The answer is wind because this wonderful riddle is playing a clever trick with words, making you think about a bird or an airplane, when it is actually describing the invisible air all around us. Let's look at the first part of the riddle: "What flies forever". When you hear the word "flies," you probably picture something with wings soaring into the sky. But the wind "flies" in a special way. Wind is really just air that is moving from one place to another. You can think of it like a giant, invisible river that flows constantly all around the Earth, both high up above the clouds and right past your face when you step outside. Even when you can't feel a big, strong gust, the air is still gently flowing, pushing tiny things like dust and pollen, and carrying the wonderful smells of dinner or flowers. Scientists who study the weather know that air currents on our planet are always in motion, constantly traveling across oceans and continents, so the air itself is always moving or "flying." Now let's look closely at the second part of the riddle: "Rests never". When you rest, you get to sit down, take a break, or go to sleep. But the wind never takes a vacation. It is always busy moving because of how the Earth and the sun work together. The wind is created when the sun heats up different parts of the Earth at different times. When a spot gets warm, the air there rises up. Then, cooler air from another spot rushes in to fill that empty space, and that rushing air is what we feel as wind. Since the sun is always shining on some part of the Earth, and the process of air heating and cooling is constantly happening, the wind has a never-ending job. It is always traveling and changing, which makes the wind the only thing that truly "Rests never." It is the powerful, invisible breath of the entire planet, always on the move and never stopping for a moment.