Because to me, they seem like de facto "Agree and “Disagree” buttons, whether or not it was the intent.
This is precisely how I use up and down votes.
I would also, as an example upvote something I think was incorrect if the was an insightful reply to it I felt people should see.
To clarify, I’m not talking about a troll post with a clever “dunk” reply.
Trolls always get downvoted.