Catra is still with the Horde because she seeks validation, did you jump off before season 2? It's explained there.
Catra has always struggled with being Adora's shadow, the only thing that kept her from suffering about that was the fact that Adora was always by her side and genuinely appreciated her, when Adora left, the message Catra got was "She saw you suffering all your life and didn't do anything, but now that a bunch of randos are having a bad time, it's her breaking point, you meant nothing to her", she feels like if she leaves the Horde she'll be back to being Adora's shadow, and she'll never be able to prove herself. >So does Entraptra think they left her or not? She keeps flipflopping between 'oh I'm sad they left me' and 'oh I don't care'
That is because she is probably autistic and as such moodswings and short attention spam aren't that rare, in the Horde she has everything she cares about, technology and science, and the best part is that there's no morality getting in the middle of it.
Also, Catra and Scorpia seem to treat her alright, particularly Scorpia, she is cozy after discovering that they weren't threatening to her. >Glimmer doesn't learn a damn thing.
Her character gets better in season 2 too.