Right, there are simply not enough dolphins, and there is simply not a good enough incentive for dolphins to 'properly' curate content.
Why should a dolphin upvote some article, when they could instead use the vote power to upvote an article they know whales will upvote, and thus give a massive curation reward?
Then indeed, they move on to trying to reap rewards... causing a selfish behaviour.. and perpetuating the tragedy of the commons.