Very interesting question. I am not 100% sure, but regardless of whether the lens is round or rectangular, the image produced should still be in the form of a circle (thus the term "image circle" of the lens). It's also easier to attach round filters to round lenses, and round focussing rings as well.
I am not an optical design engineer, but here are my guesses:
1. Circular lenses are the easiest to design and manufacture.
2. If the lens need to be rotated during operation, such as focusing and zooming, then circular design makes more sense.
3. Even if focus and zoom mechanisms can be designed to work by shifting the lens elements without rotating them, the mechanism will most likely be bulky, costly and difficult to manufacture.
4. If you want a point of light source that is out of focus to form a circular blurr spot on the film, then the aperture has to be circular. The surfaces on the lens elements that light actually passes through would then, I guess, be circular. Each rectangular lens element must then be larger than the circular area that light passes through. To acheive the same optical performance as any circular design, the rectangular lens muct then be larger, than the circular design. This is just like adding corners to a circle, and the added area is just waste material. (Imagine the bokeh of a lens with a square aperture, wonder how it would be like?)
Just my guess. Hope Ian can shed some light on this topic.
I believe the lens is first manufactured as a circular lens, then grinded to the rectangular shape for eye glasses. The optical system is quite different between a human eye with glasses vs a piece of film with the lens assembley to form an image on the film.
From both a design view a circular lens is the least complex shape to work with, as if the lens is accurately made (approx 1/2 wavelength of white light or better) then the lens will show equal levels of chromatic and other abberations regardless of how the lens is rotated around it's optical axis.
Camera lenses by their very nature tend to have at least one moving lens element and in many lens barrel designs this element rotates while moving, thus requiring the use of a circular element. Also assembly is easier with circular elements as the manufacturer can use round threaded bushings to hold lens elements at their correct spacings and the orientation of the element when placed against the bushing or housing is immaterial.
Rectangular lenses can be made by using two basic technologies, moulding and cutting a circular lens to shape. (see below for info on how glasses are made).
Rectangular and square lens elements often have some pretty nasty chromatic abberations near the edges along with coma and other fun things that wreak havoc on image quality.
Spectacle lenses (rectangular/square and strange shapes) are cut to shape by the folks at the optometerist on a lens shaping machine, which is essentially a cam following grinder.
In the case of sunglasses that are mass produced the lenses are injection moulded plastic in most cases.
It should be noted that in the case of Maksutov and Schmidt Cassegrain designs often used in "Mirror" or compound lenses that the on axis placement of the lens elements is critical to the system performance.
Added Sat 15/06/02
I forgot to mention that rectangular or square lenses have real problems in being mounted accurately as well. While they are fine for low end visual use magnifying glasses they are not exceptional. High end optical systems employing such as some tank periscopes that have rectangular elements cost a bomb .. eg: 1000-3000 USD per lens element.