Why does FM sound better than AM?

Define "sound better", please. As we all know this is something subjective - mostly everyone has individual preferences. I would have found it better if the title was "Why is frequency stable in spacetime and amplitude not (which can be verified so easily by listening to audio radio)".

The most familiar definition, though not spelled out in the article, is audio fidelity which is the degree to which the output reproduces the input. It's fair to take this definition as a default, or implied by the article. In this specific case, frequency response and signal-to-noise are both fidelity measures.

Also, some of the comments did a better job than the article of explaining things.