Lots of practical reasons why a lens is round, but most are simple engineering and practical convenience. With zoom and focus requirements, many parts in the lens need to turn, and things just works so much better when the outer carrier is round. Round is also a very simple form to make, and is structurally remarkably stable. Parts can be threaded together rather than flange bolted as required if square. Forming and machining techniques are cheaper if based on round shapes as turning is a common and cheap technique. The mounting flanges would be a bit awkward if not based on round twist lock. So round it will be. As far as optics are concerned, they could just as we'll be any shape and would still function. Although bokeh might be a bit strange with square aperture blades, this is one area that would be a lot simpler if only square form used...
Quote: =vonBaloney;2906429...
A better question might be, "Why isn't the sensor round?" ....
Or to my mind, "why isn't the sensor square?", would be a more pertinent question. For a little more silicone wafer you can save the cost of a vertical grip...
But I think the mirror and body conformance with old standards sadly prevents this approach.