Why is this the answer?

The answer is time because this powerful riddle describes something that is always moving, never stops, and is stronger than anything in the whole world. Time is the one thing that affects every single object and every living creature, from the biggest mountain to the smallest ant. Let us look at the first clue, "Mountains will crumble and temples will fall." This clue talks about things that seem super strong, like huge mountains and ancient temples made of stone. When you see a mountain, it looks like it will last forever, right? But time keeps going, year after year, for thousands of years. The wind, rain, and cold are all helped by time to slowly wear down even the hardest rock. It takes a long, long time, but time wins in the end. The great, solid temples that people build also eventually fall apart because time keeps passing. The riddle wants you to think about how everything changes over a massive amount of time. Now, let us think about the second part: "And no man can survive its endless call." This clue is about us, the people on Earth. We all have a certain amount of time to live, and no matter how strong or fast a person is, nobody can stop time from moving forward. Every second that goes by, we are getting older. When the riddle says "endless call," it means that time is constantly passing and one day, every person's life will come to an end. It is a natural part of life's journey. Time is invisible, you cannot catch it, and you cannot build a wall high enough to stop it. It is what makes things happen very slowly, like the crumbling of a mountain, or very quickly, like the tick-tock of a clock. Since nothing, not even the strongest person or the largest rock, can defeat time, it is the perfect answer for this very wise riddle.