Because I want to see ARGUMENTS because that's what people live through in real life. You do a thing you deem right only to have others disagree with you, that makes you doubt your motives or you are sure they are sincere and argue why against people that disagree. If I want a unnuanced evil VS good story I would watch horror movies with people VS monster insects.
Add to that that antagonists are usually the more interesting characters for playing the role of those that set thigns in motion whereas heroes usually only react and have no cause or goals aside from keeping the status quo. I want visions, and since it's fictional anyway I want to see what IFs. What if this utopia/dystopia existed, what if a villain manages to defeat death, what if blahblah. Again, if I want a simulation of the status quo and people wanting to maintain it I would observe the boring people around me. No need to delve into fiction for that.