Because goddess Kali is the angriest women in the world.. actually she is Parvati(the wife of lord shiva) when she gets angry and very angry she forms in Goddess kali....if you observed her then you see that in one hand she has a bowl and sword in other....when she killed anyone and took his blood in the bowl then that thing reborn and their is only one way to keep her calm down that is lord shiva's chest.... Plz take it as a brainest...