Because sound is thought in the form of waves (sound waves). Sound waves have the characteristic of transverse propagation, and almost all waves have this similar characteristic. But because you can't see it, you can't analyze it directly. But we can observe with the help of tangible and similar things, such as water. What will happen if we throw a stone into the mirror-still water? I saw the waterline (that is, waves) scattered around. If there is an obstacle in the road, it will go around. However, if there are bigger blocks, some of them will be "bounced" back, still in the original form. The sound was also blocked back by the mountains in the valley. So we heard the echo.
This has nothing to do with the speed of sound, but only a matter of time. S=v*t