1) It’s illegal.
2) Even pristine audio is somewhat degraded since YouTube uses mp3 encoding of any audio submitted. In many cases, it’s an mp3 of an mp3 of an mp3….. and the audio gets worse every time it’s re-encoded in this manner.
3) But if the above reasons are not enough to convince you it’s a bad idea to extract audio from a YouTube clip, then realize this: In many cases, what you are listening to on a YouTube clip is the room in which the audio was played. Even if the clip doesn’t have audience noises, like coughing, moving around in seats, and other assorted venue noise, the audio is playing back over a sound system, and being picked up by a microphone, along with all of the reverb, reflections, and echoes of the room in which the music is being played. All of these things combined downgrade the audio, sometimes a little bit, and sometimes to the point of pure garbage. But it’s never as clear as it could be. And once degraded in this way, there is no practical way to restore it, except to go back to the original source. That is why, when creating music edits and remixes for dance teachers, Squirrel Trench Audio always goes back to source audio whenever possible.
If you want GOOD, CLEAN audio, DON’T get it from YouTube!