Why is this the answer?

The answer is time because this classic riddle uses big, strong words to describe something that is invisible, yet incredibly powerful. This riddle is talking about how everything in the world changes, decays, or wears down as time passes, even things that seem very strong and long-lasting to us. Let's look at the first few lines: "This thing all things devours: Birds, beasts, trees, flowers." This means that time is what makes all living things grow old and eventually return to the earth. Time causes a tiny seed to grow into a tall tree, but then it also causes the tree to eventually decay and fall apart. Time is the force behind the entire life cycle of birds, animals, and flowers. Nothing stays the same forever when time is involved. Next, the riddle mentions how it "Gnaws iron, bites steel; Grinds hard stones to meal." Iron and steel are some of the toughest materials people use. But if you leave a steel fence or an iron tool outside for many, many years, you will see it rust away and become brittle. Time, with the help of weather, slowly breaks down the metal. Think about how the ocean wears away even the hardest rocks on the shore, grinding them into tiny grains of sand over thousands of years. This slow but constant power of time makes it an unstoppable grinder that can break apart anything, even hard stone. Finally, the riddle describes how this thing "Slays king, ruins town, And beats high mountains down." This is about how even the greatest human creations cannot escape time. Powerful kings live and die, and their kingdoms crumble. Busy, famous towns become old ruins as time moves on. In fact, even the biggest mountains on Earth are slowly flattened and worn down by the rain, wind, and the millions of years of time passing by. Time never stops, it never sleeps, and it is the strongest force there is because it changes absolutely everything.