Because, a ducted jet looks more scale and has the appeal of more closely replicating a true turbine (no external blades). In fact, most modern jet engines are more similar to ducted fans than turbojet engines are... At least, the high-bypass turbofans used in airliners are. They essentially use a small turbine engine to drive a large ducted fan... most of the air entering a modern airliner engine does not burn with fuel or go through the combustion chamber at all...
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