Once upon a time, screens were 4:3, video equipment produced 4:3 video, YouTube videos were 4:3, and all was well.
Then the industry said: Let there be 16:9. And there was 16:9. And the chaos began.
It began with 16:9 TVs which deformed 4:3 material to fit the 16:9 screen, and players that allowed to configure a 4:3 screen for output, but deformed 16:9 material to fit the 4:3 screen instead. But not only that, even TV stations did deform 4:3 material they used in 16:9 broadcasts (like old 4:3 recordings showed in new 16:9 news). It continued with pure 4:3 broadcasts no longer being broadcast in 4:3 (although still technically possible and understood perfectly by any 16:9 TV), but 16:9 with black bars left and right, which on a 4:3 TV causes a black frame around an image that is much smaller than necessary.
But at least those were professionals, so while their decisions were not always what you would wish, what they did was at least halfway sensible. But on YouTube, you'll find an even worse situation: 4:3 material converted from 16:9 to 4:3. What this results in is a square image surrounded by black bars left and right, and certainly again the ugly distortions which non-proportional rescaling brings.
In short, since the advent of 16:9, there are masses of terribly distorted videos. And strangely, I've not even seen a single comment about that terrible distortion (but maybe having a minimal sense for aesthetics is strongly correlated with having no desire to get an account with Google — which actually is the main reason why I didn't leave such a comment on such videos).
OK, granted, I no longer see any comments on YouTube, so I can't tell if it has changed (I guess it's related to Google+ integration). But I'd expect such comments to have been made early on anyway.