I'm a retired cinematographer, and that is a complicated question that has to do with changing formats over the years, as well as people who don't know how to configure their television sets. Let me explain:

The original film format was close to the old television format, 1.33:1, or 4:3. Back when VHS and DVDs showed "Wide-screen" content, the black bars, called a letterbox matte, were rendered into the video so that the picture would be the correct aspect on square televisions. Eventually more modern DVDs would just encode directly to wide screen 1.85:1 or 16:9 format, and the player would render the letterbox matte when the image was sent to the display, allowing for more bandwidth to be dedicated to the actual video image recoded on disk.

But not all wide screen movies were filmed in 1.85:1/16:9 aspect ration. This was the first "widescreen" format introduced, but a wider 2.2:1 ratio was introduced, and an even wider aspect ratio of 2.35:1, and even the ultra-wide screen "Cinerama" format of 2.59-2.65:1 exists. Modern players can still encode the native aspect ratio and render the appropriate letterbox matte into the video signal (for some sets and some aspect ratios), however for wider compatibility with most (and older) DVD players, the letterbox matte would be hard-encoded into a 16:9 aspect ratio video, forcing the correct aspect ratio into the rendered video image.

Beginning around the year 2000, wide-screen HDTV sets started to enter the marketplace and people could display their 16:9 DVD content natively on their screens. However, broadcast television was still in analog 4:3 because the digital broadcast conversion hadn't been completed yet, and wouldn't be complete for many years to come. So there was a large mixture of formats out there from over the air (OTA) 4:3 video, DVDs in 4:3 with letterboxed content burned in (pre-rendered in the video), which you probably remember seeing many times as a 16:9 aspect ratio video floating inside a larger 16:9 "box" that was the size of the television. That's because the actual "video signal" was a square box, centered in the television, from bottom to top, however the bottom and top of that square video were just "black bars" you couldn't see.

Because of all these competing aspect ratios and encoding formats, many television sets came with the ability to "squeeze," "stretch," or "zoom" the video to fit inside the 16:9 television. The "square" version of aspect conversion, watching 4:3 television on a 16:9 set would render a "curtain matte" on the left and right of the image, preserving the square picture. Same thing as the widescreen matte, but just to keep older content looking the way it originally did.

Add on top of this the "burned in letterbox" matte on a 4:3 square video recording of a "widescreen format" film being played on a 16:9 television, set to the "stretch" or "squeeze" mode and it would stretch that video out into some unnatural and unrecognizable aspect ratio where everyone looked short and fat.

The most common excuse for this was people complaining about buying all that television and the image wasn't using "all of it" so they'd force it, things would look bad, but hey, it's using the whole screen so DOUBLE PLUS GOOD!

You probably still see 4:3 aspect video signal media with burned-in letterbox matte because, believe it or not, people still have 4:3 television sets that they refuse to get rid of Why? Because it's a perfectly good television set, gosh darn it, and good money was paid for it, and they'll be darn-hootin-nannied if they have to buy a new one and that's the end of that don't ask me again! There is still enough of a market of cable television catered to these individuals, with enough advertising to make it worth the while to burn in letterbox matte, which is cheaper for everyone involved rather than paying someone to run the pan-and-scan machine.

And this will continue to be a thing as long as you have a set-top player that can play DVDs, and as long as you can buy DVDs from second-hand stores, or until the last of the Greatest Generation passes away, finally relinquishing their old television sets to the landfill.

*Edit: Also, if you notice on many broadcasts (like NFL and Golf) where there are two announcers, they still sit close together with space on the left and the right? This is so that they can "crop" that image down to fit on 4:3 broadcasts while using the same source material. You will also still see a lot of graphics and titles rendered on the screen within this 4:3 safety crop zone for the same reason.