Because evil is a fundamental (abstract) concept enmeshed in the moral, spiritual, and religious domains of life. If philosophy is anything it is the study of d

Because evil is a fundamental (abstract) concept enmeshed in the moral, spiritual, and religious domains of life. If philosophy is anything it is the study of deeply interwoven fundamental abstract concepts. Because the problem (what is it? why does it exist in the first place? if there is a god and god is good why would/could there be evil at all – etc.) of evil is felt to be of such significance it would be noteworthy that philosophy did not have the tools to take it apart and inspect it from all angles. It would indicate not that our conception of evil was flawed but that there was something majorly lacking with the discipline of philosophy. That'd be my take on it. The same would go for any fundamental abstract concept.