It's like books - another device like smart phones, computer and tv that are mostly about the information they contain rather than the form of the product. Those have also been rectangular with the same form of pages and a binding at the end for hundreds of years because people want them to read, not to have some weird shape or binding mechanism.

Non information products still have a variety of design forms. People say cars are bland but a Mini, Ferrari F80, F150 pickup, Range Rover etc have pretty different forms.