I dont see a single reason why we shouldnt have downvotes as well
It takes the entire spectrum of disagreement (from a well written post with well supported points, to a chimpanzee banging a femur bone on an empty oil drum) and compresses them down to a single bit. In the industry* they refer to that as signal degredation.
*Not my industry but, you know, an industry.
Counterpoint to "upvotes are the same thing, but with agreement instead": people love to hate on shit, for free, all day, for its own sake. Expressions of support are more genuine across the board.