An exhaust manifold can easily go over 800 to 1000 degrees. For instance, an engine functions via small explosions that can be several thousand degrees, the engine block itself is cooled. So by the time the exhaust gets to the back of the car it is (by law) not at a dangerous temperature. The heat has to go somewhere, and that somewhere is the manifold which is only cooled by the air flowing around it. Most heat from an engine is lost in this way.