The whole resurrection goal feels really odd to me. A lot of the other cupids seem like they've been in this industry for a long time. Is the amount of karma each cupid need to collect different or something?
Well, as Kanna said, they also use their karma to buy their equipment and ammo. Also, sometimes Cupids have the same targets and a wrong choice results in no karma. So, it's a system set up to get them to work as long as possible, and at some point, do you even want to be resurrected, the people you care about might have aged way past you, the world changed so much you don't recognize it anymore, and what do you do? As a kid, where will you live, how will you pay for food, who will take care of you? You are going to be an orphan or homeless. To be eternally young and not have to care about the struggles of life and go bang bang boom without any collateral damage? sign me up.
Did the author think about all of that stuff or did they just want to write a cool action yuri? I don't know.