In my opinion the criminal Lazarus group has a significant advantage over any other hacking group: it's based in North Korea, a designated terrorist state. Therefore, they are protected by its insane leader, and the law cannot reach them.
As for other groups, like Loopscale, it's extremely difficult to cash in the stolen funds, so it's better for them to return them and receive a 10% reward than to be arrested and imprisoned. Furthermore, they'll gain notoriety, and companies may hire them to find vulnerabilities in their products in exchange for a large reward.