Some whale like thejohalfiles is not going to downvote a stupid bot like markboss at 100% voting weight. He has NO monetary reward in doing so. If he downvoted 10 of markboss's posts at his full voting power and weight, markboss's account would go negative reputation and would be pretty much useless to him. But thejohalfiles would just be wasting more than $1300 in upvotes because markboss will just make a new spam account! See, downvotes should be rewarded

If downvotes were rewarded, hundreds of minnows would jump on the train when a whale downvotes. This would effectively sink every spam account out there. Whales are concerned with the health of steemit as it is their income, so they wouldn't downvote good posts

there is little incentive to downvote good comments. At best, some stupid minnows might do so. The whales and dolphins would receive far less than their vote's full value if downvotes were rewarded and pooled

Its described well in The Wealth of Nations by Adam Smith. Search up "the invisible hand"
Downvoting needs rewards

I doubt that most people would downvote good content because it gives them enemies on steemit. Those who make good posts almost always have more leverage than the spammers do on steemit