It has generally reduced the signal-to-noise ratio of writing, but the signal-to-noise ratio has been absolutely terrible for such a long time we’ve all adapted to better signal detection.

Because it made noise look more like signal. Essentially, that is what AI is. A noise2signal generator, but you specify how a signal should look like. Then, it makes the noise into your specifications. If you specify "make it look like good writing", it will look like good writing, but it will still be noise. It won't be good writing.