How come? Or how come it doesn't make it a requirement to POST in the thread clearly stating your argument/reason for up/downvoting in order to down/upvote someone?
If I were to venture a guess, the vote system, while helpful for drowning out people spamming shock sites and other OBVIOUS trolls, is generally quite toxic as a whole, and doesn't add all that much.
For example, back when Diana was first released and clearly over-the-top OP. There were more than a couple of posts going into rather accurate detail exactly why Diana was OP. Most of them got like 8 downvotes without a single reply. Quite a few threads accurately explains why a champion, like RELEASE Zyra was OP, but was downvoted to oblivion by people who bought her on release.
The intent of the votes were obvious, and they're clearly detrimental.
And then there are the all too common cases where people are asking completely neutral and innocent questions...and they get downvoted to oblivion. A person asking stuff like "what is Olaf's weakness supposed to be, I can't find it?" will receive all the downvotes from Rengar players even if he didn't ask for any nerfs.
What would happen if the up/downvote were revealed. Like "You were upvoted by __,__, and___" and "You were downvoted by ____,____, and ___". Or heck, make posting and explaining your choice of vote a requirement before voting.