Kali is the embodiment of the Goddess in her fiercest form. The fact that she is single, having no husband, keeps her fierce.
When goddesses are coupled with male consorts, their shakti is cooled, and they become mild in disposition.
Thus Radha (Krishna’s divine lover), Parvati (Shiva’s consort), Lakshmi (Vishnu’s consort), Sita (Rama’s consort), and Sarasvati (Brahma’s consort) are all peaceful goddesses.