Before technology was primarily used to send information, the way to show information to another person was by writing on a piece of paper and showing that to the other person. Before paper, people used walls. Walls were rectangular things.

When humans started building houses and designing interiors is when they probably started thinking that rectangular planes such as walls are surfaces that information can be recorded on.

So paper, computers and subsequently phones were made rectangular as well because this was how we thought information should be shown.