Probably like they themselves fighting an even bigger threat, or they playing a gamble with the evil god for entertainment or something like that.
Their power is too much and if they interfere directly then reality will crumble or something like that. There's also the troupe "We are Gods and you are our plaything, so fight that guy while we enjoy seeing you suffer and struggle"
Cause and effect. There are multiple forces wrestling for control over a world, good gods, evil gods and etc. The more you intervene, the more the opposing forces can interfere. So creating a "hero system" gives more flexibility as the god doesn't have full control of the hero and the hero generally has to grow stronger over time. Aka, the more restrictions, the less the other side can intervene.