Because it makes for good theater, that's why. If Hulk Hogan didn't try to kick, punch, and body slam Andre the Giant, the professional wrestling fans who bought a ticket to the show might start to believe that Hulk Hogan and Andre the Giant are actually friends and disinterested in hurting each other. They might start to believe that it's not real, and that can't be allowed to happen.
The owners and promoters backstage make sure that the illusion looks as real as possible, and the same is true in Washington. But instead of kicks, punches, and body slams, the politicians attack the other party's legislation and "try" to get it repealed or overturned. Of course, they pretty much never succeed, but it's important that they appear to try, otherwise the voters (professional wrestling fans) might start harboring ideas that it's all fixed.
Wake up, dude, politics is theater. It's a show. It's a distraction. There's no real struggle taking place in Washington. Any struggle you see taking place is choreographed for the fans and cameras, just as the struggle between professional wrestlers is. None of it is real.