Why are computer screens rectangular?

Because the human field of vision isn't square, you see more stuff on the right and left, then up and down, so when they make standards for monitors and tvs, they make them have a wide aspect ratio. Sometimes programmers like having vertical monitors so they can see long documents of code better.

As opposed to...?

As tvhargon said, the human field of vision benefits from a display that is wider than it is tall. A circular, triangular, pentagonal, hexagonal, or any other geometric shape for that matter would not only be very difficult to manufacture, but would not take full advantage of how the human eye works, and would result in inconsistent scan line sizes for the video signal going from the computer to the monitor. It is true that the original cathode ray tubes were circular, back before there were any TV standards and when computers were extremely primitive (before the day of transistor-based solid-state electronics), as they were easier to manufacture that way.

It's much easier to make web pages/applications/everything for rectangular screens rather than circular/octagonal/dodecagonal screens. If anything, they would be square, which some of the smaller ones get pretty close , but the big ones need to be rectangular because otherwise they wouldn't fit on people's desks, or be comfortable to look at.

as its the best viewing ratio

all items fit better on a rectangler screen just like WS is beter than normal 4:3 as its a nicer and easier to view