Because the smartphone needs to fit in your hand. You can think of it as raising the screen to body ratio or minimizing bezels, but really what is here is maximizing screen size. However on a smartphone you can’t just make the device bigger because it becomes harder to hold. So generally they try to reduce bezels so that they can get a large screen without the device being too large.

TVs don’t really benifit from reducing bezels (unless you are tiling them) and same with monitors (although putting monitors next to each other is more common so there is some benifit). Laptops with really small bezels are common these days and they do have the same problems as phones, getting the biggest possible screen in a smallish enclosure size. Although the bezels here are already relatively small even for old laptops, so there is less gain to be found so not all manufacturers bother pushing it to the extremes for that 0.5% improvement.