Why is this the answer?

The answer is wall because this is a very clever riddle that uses a simple word trick to make you think about a person or an animal. The special trick word here is "run." When you first hear the riddle, your brain probably pictures a runner moving quickly with their legs or a dog running in the park. But a wall does not have legs, so that can't be the kind of running the riddle means. That is the first part of the puzzle that tries to fool you! The secret to this riddle is that the word "run" has another important meaning. When something *runs*, it can also mean that it stretches out or extends in a long, continuous line. Think about a long, straight fence or a border line on a map. You might say the border "runs" from the top to the bottom of the map. It does not mean the line is moving; it means the line is long and unbroken. The wall around a city works exactly the same way. In many old stories and times, big cities had giant, thick stone walls built all the way around them. This wall was there to protect the people inside and to mark the city's boundary. Even a wall on a regular house, or a tall fence around a neighborhood, stays perfectly still. This matches the second clue, "but I never move." The wall itself is stationary; it never gets up and walks away. However, because the long line of the wall goes all around the perimeter or edge of the town or city, we say that the wall "runs around the city." It is one strong, long border that defines the shape of the city, stretching for many miles from one point to the next. The wall is the boundary line, and that line "runs" everywhere, even though the bricks themselves are cemented firmly in place. This clever riddle uses the size and length of the wall to trick us into thinking about movement, making it a wonderful play on words.