Because to me, they seem like de facto "Agree and “Disagree” buttons, whether or not it was the intent.
Interesting to read the comments, I was unaware anyone gave mich if a shit about posts, let alone how others might vote on them. I mean I don’t actually know anyone here.
I do hit the upvote occasioanly if a post was helpfu/usefull… to me. Conditioning is the only real explanation I have for that behaviour though.