Because a god should act as a neutral party. Neither side is actually good or evil to them. Both sides are simply trying to exist. Helping one side directly leads to jealousy on the other side and it only results in more conflict between the two sides until one side eradicates the other further down the line. A god isn’t really interested in the eradication of either side, but rather watching the way a scientist might watch ants in a conflict. The god would choose to use a third party as a balancing piece when the sides are unequal to stabilize the two sides, thus creating a delicate equilibrium. This is because competition between species leads to them evolving and becoming stronger. Those the god observes would be nothing more than a science experiment to pass their time, something interesting that might alleviate some of their eternal boredom. Though the god may grow fond of all his creations and love them all equally, those creations may come to hate that god thinking they’re viewed as nothing more than play things for the god’s entertainment once they gain awareness. If a god reveals himself to his creations and shows impartiality to one side, he will be hated by the other side. A god may simply not wish to be hated by those he created. Thus, rather than be hated directly, use a hero as the sacrificial piece to redirect hatred away from oneself.