It is a nice aesthetic right? The short answer is that in volume, injection molding is cheaper. That said, you can buy a wide variety of "standard" extrusion products if you want to build a custom case for something. This happens fairly regularly in custom instrumentation. Most recent example I saw as a tranceiver that used 80/20 quarter round extrusion (#1517) on the corners and sheet aluminum for the sides top/bottom. The front was likely laser cut or milled aluminum. I don't think it's been opened up yet for service so I don't know if there were any additional structural supports inside.