Most athiest claim to not believe in God, because it's not scientific.
There is absolutely nothing scientific about the belief in karma. Like religion, it is comforting to believe that everyone ultimately gets what they deserve (in regards to how they treat others), but most evidence is actually not even nuetral on the topic of karma, but contradicts it's existence. People who are mean and rude to others, take credit for others work, etc. are often the most successful.