I think it's because we've evolved in nature to appreciate natural sounds, which always have distortion, harmonics and noise. There's nothing that would genera

I think it's because we've evolved in nature to appreciate natural sounds, which always have distortion, harmonics and noise. There's nothing that would generate perfect sine/sawtooth waves or perfect, undistorted echoes. So anything else just sounds unnatural/robotic. I think it's the same with film grain, looking at flat perfectly polished surfaces can seem boring and unnatural to the eye.