All TV's . Its kinda like blowing up a small picture. It just gets grainy. Upscaling tries to fill in the gaps. It might be ok to go a few steps up 420 to 720 to 1080 but to go further is a mess . Unless you have something big enough for th escaler to work with. This explains it better than me. LOL So plain cable TV might be 720p it has I think 777,600 pixels that would be impossible to stretch. I'm sure your TV has setting to compensate for this so those black bars are not half your TV.
Remember, physical resolution is defined by the number of pixels on a display. It has nothing to do with the actual size of your TV. A 1080p TV screen is comprised of only 2,073,600 pixels, while a 4K screen has 8,294,400. If you show a 1080p video on a 4K TV without upscaling, the video will take up only a quarter of the screen.
For a 1080p image to fit a 4K display, it needs to gain 6 million pixels through the upscaling process (at which point, it will become a 4K image). However, upscaling relies on a process called interpolation, which is really just a glorified guessing game.