Why are Airliner's Engines Shaped Circular (the 'why' of shapes)
Airliner jet intakes are round because the engines are round, and for subsonic-flight. That’s all you need as there isn’t that much pressure building up in the engines compared to supersonic flight — it’s easy and cheap to engineer, provides a clean airflow for the engine and unlike some other shape doesn’t present unnecessary frontal area that will only cause drag.
The turbofans used in civil airliners operate at subsonic speeds and don’t require any shocks for flow deceleration. The circular shape is the best for getting maximum amount of undisturbed airflow into the engine, especially the bypass section.
— Ved Joshi (S2)