Cast iron exhaust manifolds last a long time, and they are cheap to make. That is why so many cars and trucks come with them from the factory. In modern cars, exhaust manifolds are designed to fit into small spaces and live long lives. Maximizing horsepower is rarely a factor. That is why cast iron exhaust manifolds are often replaced with more free-flowing tubular designed "headers" when the vehicle's owner is on the hunt for increased horsepower and torque. Cast iron manifolds are also heavy, and can become brittle with heat cycles and age. If they crack, the check engine light will pop on, and the vehicle will likely become embarrassingly loud. Not to mention, all of those toxic exhaust fumes would be able to escape the confines of the manifold, and dance their way into the interior of the vehicle. The same thing will happen if the exhaust manifold gaskets blow out, or the mounting studs break off