The belief in witchcraft was widespread during the Elizabethan and Jacobean eras. Witches were feared and often persecuted. People believed that witches could cast spells, curse others and cause harm through supernatural means.
Shakespeare would have been aware of these beliefs and included elements of witchcraft in Macbeth to add an extra layer of tension and create a sense of unease among the audience. The presence of the witches and supernatural events would have resonated with the contemporary beliefs and superstitions of the time.
Additionally, it was not uncommon for playwrights to incorporate supernatural elements such as ghosts, witches and magical spells to enhance the dramatic impact of their works. These elements were often seen as a way to explore moral themes and add a sense of wonder and mystery to the play.
In conclusion, audiences during Shakespeare's time would have assumed that the strange elements in Macbeth, including the witches and their prophecies, were caused witchcraft, reflecting the prevailing beliefs and superstitions of the era.