People post things they believe are accurate, correct, on point, etc. If others disagree, an explanation provides a learning opportunity. Also, requiring an explanation would eliminate the erroneous downvotes we so often hear about. If others agree, who needs an explanation? Nothing to learn there. Downvotes should simply indicate disagreement, but they have been used maliciously in the past (review the related threads).
Upvotes indicate agreement and generally good vibes only. Upvotes without downvotes would go along with the ethos of 'If you have nothing nice to say, don't say anything.' Upvotes also greatly reduce the volume of 'Me too' posts.