That's a very good question. In Circe, Madeline Miller suggests that witches are made, not born. In particular, they're made by ancient society, molded by the many restraints that society places on women and their development.
Circe's father, a Titan, displays in all its ugliness the dominant cultural norm concerning the role of women. Profoundly disappointed in what he sees as Circe's womanly weakness, he wastes no opportunity in letting her know just what a let-down she is to him.
But Circe, being Circe, is a strong character and is not about to take such nonsense lying down. In this particularly tense exchange with her father, she bravely defies his insulting words and stands up for herself:
So many years I had spent as a child sifting his bright features for his thoughts, trying to glimpse among them one that bore my name. But he was a harp with only one string, and the note it played was himself.
“You have always been the worst of my children,” he said. “Be sure to not dishonor me.”
“I have a better idea. I will do as I please, and when you count your children, leave me out.”
Circe has made her mind up to do as she damn well pleases, and she's no longer prepared to listen to anything more her father has to say, even if he is a powerful deity.
That said, there are still a limited range of options available to Circe. Although she says she will do as she pleases, she must realize that it will be impossible. However, one of the few options open to her is the practice of sorcery. Being a witch will give Circe, as with many other women in ancient Greek society, the rare opportunity to hold and exert power—in particular, to exert power over men, something she would not ordinarily be able to do.