It was popular at that time. All the Curtiss P-40 Warhawk, North American P-51 Mustang, Hawker Hurricane, Supermarine Spitfire, Hawker Tempest, Messerschmitt Bf109 and many more had similar.
That kind of exhaust was tied to the engine, a water-cooled 2-line 12-cylinder (V-12) and later some 4-line 24-cylinder (H-24 or X-24) engines. So there was a row of cylinders on each side and each had its own exhaust pipe as short as possible to minimize its resistance.
Since then, anything with more than 6 cylinders was replaced by turbines and all the smaller engines are air-cooled, because in GA aircraft the simplicity of air-cooled engine is more useful than the slightly lower drag of water-cooled one (navy preferred air-cooled even in fighters for the same reason). And air-cooled engines generally have their exhaust pipes lead into the cooling air stream and thus hidden under the cowling.