Why is this the answer?

The answer is well because this classic riddle is a very clever way to describe something important that gives people water. A well is a deep hole that people dig into the ground to reach the cool, clean water stored far beneath the surface. This riddle uses three great clues to help us figure it out. Let's look at the first clue: "As round as an apple." If you look down at the opening of a well, the part where you drop the bucket, it is almost always a perfect circle. That round shape is just like a sweet, juicy apple! This clue helps you picture the top of the well. Next, the riddle says: "As deep as a cup." This is the part that tries to trick you! We know a real cup is very small, and a well is a huge, deep hole. But think about what a cup is for. A cup is a container, and its main job is to hold water or other drinks. The well is like a giant, super deep container that nature fills with water underground. So, it is like a huge cup for the earth's drink. The last part gives us the biggest hint: "All the king's horses can't pull it up." This is absolutely true because the well is not an object sitting on the ground, like a bucket or a fence. The well is the hole itself! You cannot pull a hole out of the earth no matter how strong you are, or how many horses the king has. The well is a permanent part of the ground, always ready to keep its treasure of water safe. All three clever descriptions point right to the word "well," making it the perfect solution to this puzzling rhyme. It is a fantastic example of a riddle that plays a fun trick on your mind by describing something we see every day in a brand new way.