It also didn't not need to be the case here. Zeus didn't need to be in the film. The film didn't even need to exist.That doesn’t need to be the case here. They are always taking liberties with their mythology characters.
Yet here I am, having been entertained by it and Crowe.
But let's look at the central theme of the film. The core story is about coping with an existence in which the gods are assholes, completely indifferent to the suffering of mortals and the losses they experience. The climax of the film is when Gorr chooses to resurrect Love instead of seeking vengeance against the gods. If Zeus isn't an asshole, that choice is not necessary.
Given that that's the story of the film, it kinda is necessary for Zeus to be an asshole.
Also, in any other variation, if Zeus is not an asshole, but rather he is a loving figure who sympathizes with Gorr and would act on his behalf, then that would open the door for a deus ex machina resolution in which the characters do not even need to make choices or sacrifices.