Question

Why TVs are usually made rectangle?

Answer 1

Not all computer screens are rectangular.

Some screens are square, like those with resolutions 1600 * 1200, or the old 4:3 aspect ratio.

The earlier computer screens (and the television ones too) were squarish, with an aspect ratio of 4:3 (the letterbox format). The cameras used at that time too had the same aspect ratios, so everything was well tied together. The pictures or videos taken from a 4:3 camera and the 4:3 screens on which they were displayed, fitted together smoothly, like a jigsaw puzzle.