The fundamental problem is that people will treat upvotes as agreement, even in communities who set out to make sure this isn't the case. I stopped posting at tildes.net recently, which has this exact problem with upvotes in spite of this being a cultural pain point that caused so many of tildes' users to leave reddit.

In a way it's sort of evolutionary. People attempt to propagate their own ideas or similar, and remove ideas they dislike from the meme pool, via voting systems.