actually they've down much quieter in the past 40 years. e.g. the 787 dreamliner has wavy bits on the exit of the nozzle that reduce efficiency by 1% in exchange for quieter operation because making the engine quieter reduces the amount and weight of noise insulation in the cabin

Oh of course, I didn’t mean to say that they weren’t as quiet as they need to be, only that there ought to be some obvious noise reduction opportunities once the requirement to be airborne is gone, and once the machine is being used for torque instead of thrust.