Why does cancer come back when your doctor tells you you’re cancer-free?

One of my good friends had liver cancer and she went through rigorous treatments to rid her of her cancer. It was devastating, but eventually, the good news came from her doctor indicating that she was “cancer-free.” We went out, traveled and she really felt that she was ready to enjoy life again. But several months later the cancer was back. She was bloated and depressed with jaundice. I stayed with her each day and she cried wondering why she had a death sentence.
Her pain was suppressed but she was uncomfortable and mostly sad and also angry knowing that she would die soon. She really thought her cancer would not return but I’m sure her doctor didn’t know. Why does cancer come back to so many people when they think they’re cancer-free?