On USMB there's many reasons:
-USMB tends to attracts people of extreme political views. Think about...chances are your average person in America (who there's a strong chance doesn't vote), doesn't care/bother enough to search for a forum such as USMB.
-There are tons of keyboard warriors, the Internet provides them with a safety blanket-it's easier to talk shit online than it is to a 195 lb 30 year old man who lifts in real life.
-Many on here are so emotionally involved in politics that they actually feel like they're a part of the "team"...you're not. Doesn't matter if you vote for the Republicans or Democrats-the truth is you're not in the inside club and will never be.
-These days the political climate is extremely hostile...the only thing everybody can agree upon is that the government officials are completely incompetent (look at the approval rating for the Congress).
-Anybody reading this post if you feel like I'm referring to you or are having any kind of personal reaction to it: I am.