Cats like shiny objects because something shiny attracts their attention. Once a cat investigates something, it gets curious about that object and wants to play with it.
Cats also like shiny objects because humans tend to like shiny objects such as jewelry. By playing with shiny objects, cats often get the attention of their owners, so cats quickly learn that shiny objects can get their owners to play with their cats.
To learn more about why cats like shiny objects, click here.