Because the arch is the most stable shape. People use this principle to build arch bridges. The mechanical principle in arch bridge is that the bending moment stress originally generated by load is converted into compressive stress through a horizontal thrust or most of it is converted into compressive stress. The biggest difference between an arch and a beam is that there is a horizontal thrust. If the action point and direction of the resultant force of horizontal thrust, abutment reaction and load acting on it just pass through the axis of the arch, such an arch is reasonable. However, this arch has only compressive stress. If they do not overlap, there will be a certain bending moment stress. But in general, the bending moment of arch is less than that of beam with the same span. We know that the compressive properties of stone are good, but the tensile properties are poor. Through the principle of arch, we can make full use of the compressive properties of stone with good compressive properties and avoid the shortage of tensile properties. Therefore, most bridges are designed as arcs.