Why are cats so fascinated with anything hanging and moving lol

Every toy I introduce, they are usually very cautious of.. this moving mouse, with the sound it is making on the floor, they used to be so scared of it and would just watch while hiding even though they are dying to catch that hanging toy lol .. usually it takes them a day or two to start getting closer. More courageous kitty would usually start plying with it, and then the other kitty would follow another day or later