Its so that acrylic panels can be bought as simple rectangles, easier to source and order with simplified cutting from suppliers, then the more complicated shapes of the enclosure cut by hand from coroplast, avoiding the need for custom cutting.
Tho many opt for properly cut ACM panels, the original design was focused on being able to do it yourself with minimal tools and compatible materials.
Originally they are meant to be cut form coroplast (kartonplast) and similar material. If you have that size of cutter available, you can of course make large one-piece panels. It will help insulation and rigidity.