Why do meridians have the same length?

Meridians are the lines on globes and maps that connect the two poles of our planet. They are all the same among themselves, because each meridian is equal to half the circumference of the Earth, and they all leave from one point, and also meet at the same point. It turns out that all meridians are the same, in contrast to the parallels, which are parallel to the equator, and therefore, the closer to the pole, the smaller they are.