I'd guess that the airflow through the engine is greater than the output of the chimney. So if you put it directly over the chimney, it'd draw more from the chi

I'd guess that the airflow through the engine is greater than the output of the chimney. So if you put it directly over the chimney, it'd draw more from the chimney than the power plant is designed for, throwing off efficiency. With the jet away from the chimney, it will naturally draw from surrounding air without messing up the exhaust flow.