...because, sadly, it works.
Potential supporters of your opponent might be dissuaded from going to the polls. There are some people who will never vote for you, but a bit of negative advertising about the person they might vote for could make them stay at home.
Or you might provoke voters to give you their vote to stop the scoundrel from being elected.
That's why all kind of outrageous assertions are being made - and there are no "Queensberry Rules" - Democrat X "is corrupt", Republican Y "is reckless". Candidates have "either profited off victims of the September 11th terrorist attacks or refused to honour them on the floor of the House of Representatives". All these examples come from the Wisconsin contest for the US Senate, but I'm seeing similar allegations aired frequently in Virginia.
Candidates, their campaigns, and "independent" groups do it - because it works, and there is no punishment. Romney keeps repeating the claim that Obama went on an "Apology Tour", despite the fact that all the "Fact Checkers" have repeatedly disproved the allegation. It's a technique the Nazis knew how to use - the Big Lie. The more outrageous the better.
Until campaigns have to pay a price - they will carry on doing it. Individual voters need to fight back - tell the candidates "lie about your opponent - and you forfeit my vote". Perhaps it's time to tear up the "Don't tread on me" signs - and start wearing badges "Don't LIE to ME"
How do you think this tide of mendacity could be reversed?