On your DVD player, make sure that it's display mode is set to 16:9. If it is set to 4:3 Letterbox or 4:3 Pan & Scan, then you will always get a 4:3 picture with black bars on the left & right even when playing 16:9 movies. Most DVD players I've seen are usually configured to 4:3 Letterbox by default, so this is likely the culprit.
Most recent movies on DVD using 16:9 aspect ratio, so if every DVD you play has black bars along the left & right, then your DVD player is most likely set in one of the 4:3 modes. The best way to check is to find a DVD with '16:9' or 'Widescreen' written on it, as this should definitely fill your TV's full viewing area if the DVD player & TV are correctly set up.
For your Cable or Satellite box, make it is also configured to 16:9. Otherwise, non-HD broadcasts in 16:9 will result in a 4:3 picture also.
Once you are sure your equipment is configured, the TV's wide or smart stretch mode should work better when viewing older 4:3 movies and other content in the legacy 4:3 format.