Bezels are important to frame the screen content from functional perspective. Think of it like a frame for the art work. Black bezels occulude the distracting edges and background noise from the screen and you have always have a constant black border. They can be made to look nice as well like some of those Bang & Olufsen TVs[1] and Sony Trinitron Professional monitors [2].
Contrararily, there is no reason to the opposite besides "aesthetics". Can you think of any?
Alas - the momentum behind bezels is so massive, it is impossible to reverse this trend. Same with "borderless" trends in UI/UX.
[1] http://www.extravaganzi.com/wp-content/uploads/2010/10/Bang-...
[2] https://www.proav.co.uk/media/catalog/product/cache/1/image/...