If you think about it. If you want a 4:3 picture to fill a 16:9 display. There is basically just 2 options.
Stretch it sideways so becomes 16:9, this makes round objects wide ovals. Basically Twiggy looks like Miss Piggy 
Scale it in same aspect ratio till it fills the screen sideways, losing a large amount of the picture pixels top and bottom. As 4:3 is generally poor quality doing this reduces the amount of original pixels that are used to fill the screen. This makes the picture even worse quality.
To complicate matters some broadcasters make 4:3 content by adding black pixels left and right to create a 16:9 image. It looks 4:3 of course because you can’t see totally black pixels.