Because no one actually cares about it. No one's actually read the Iliad, People just know they're supposed to have read it at some point. And have you read it? You'd have to do some ridiculous near-vandalism to turn it into a mainstream video game like what they did to Dante's inferno. Best you could hope for is like a game based on those weird side quests Achilles does near the middle when he steps away from the war