Because to me, they seem like de facto "Agree and “Disagree” buttons, whether or not it was the intent.
It really depends on where you use them.
In some communities I’ll use upvotes as a way to track which posts I’ve seen.
In news communties I always downvote tabloid sources and clickbait headlines, regardless of content.
But when it comes to the comments, it’s mostly an agree/disagree button.