Why is this the answer?

The answer is time because this amazing riddle is like a secret poem about the biggest, most powerful thing in the whole universe. It wants you to think about something you cannot see or touch, but that changes everything around you every single second. Let's look at the clues one by one, like a detective trying to solve a fun mystery. The riddle starts by saying time "builds things up." Imagine a tiny acorn in your hand. Time is what allows that small seed to slowly, patiently, grow into a huge, strong oak tree. Time helps people grow too, letting you get bigger and stronger every day from a baby to a third grader. Next, the riddle says time "Lays mountains low" and "Dries up lakes." This is a tricky clue because mountains feel so solid and huge. But over a million, million years, the wind and rain slowly wear them down. Time makes that possible. Rivers and lakes also change because of time. Sometimes they fill up with water, and sometimes the sun and the passing of time cause them to dry up completely. The way time works is that it causes change to everything, even the biggest, strongest things on Earth. The next line says time "makes things grow," which is a perfect clue. Nothing can grow without the passing of days, weeks, and months. Time is the road that everything travels on to become what it is meant to be, from a small kitten to a tall sunflower. The last two lines tell you the biggest secret of all. Time "Cares not a whim about your passing" and is "everlasting." This means that whether you are happy or sad, or whether people are on Earth or not, the clock keeps ticking and the moments keep moving. Time never stops for anyone. It went on for a very long time before you were born and it will continue for a very long time after. That is what makes time the answer; it is the greatest, never-ending force for change in the world.