Probably because no one has ever really attempted to make it make sense.
Fanon-wise, though, I usually put it down to the mirror effect. Circe was misused and abused by men, and responded to that mistreatment by becoming a misanthropic, unbalanced predator; she doubtlessly justifies this to herself as "of course I'm cruel, how could anyone not be, having been through what I've been through?" Yet here are the amazons, who were misused and abused by men and responded to that treatment with grace, poise, and a solemn commitment to be better, who are living proof that Circe's behavior and Circe's misery are her own choice. That's gotta provoke some bile. It's no wonder she was so delighted by Antiope's splinter faction, and so pissed off when it looked like they might make peace with Athens - they were proof that she was right and even the amazons had a breaking point, and she'll keep messing with them as much as she has to to maintain that proof.