Nikonian72, like many thing in life the reason may be based on odd random and/or inadvertent happenings. Someone sketches a sensor on paper as a concept, not really caring about L-W ratio. The technician doing the work measures and so the ratio never intended is set and copied or film ration is used and where did that ratio come from ... a mechanical limitation?? Thing happen ... Howie Mandel said "cause that what we do"

Then, aaaaha, ratios have to do with the human eye... aaaa er aaaaa, that is a complicated thing... but lead to an interesting article that I printed and will read deeply in the near future.
http://bankslab.berkeley.edu/publications/Files/horiz_vert_disparity_slant99.pdf

I crop to 8x10 (and print at roughly 7.75x9.75 to fit mat) because it prints fine on 8.5 x 11 paper and I can see it at a distance... and that is what the Camera Club requires. Then if I want to get a batch of 10cent prints at the local Walgreens they print 4x6 oops and crop if I do not add canvas to the 8x10 = 4x5 crop.

At this stage of the game we are confronted with many sizes and ratios that may, in some day in the future, be more logical and homogeneous. Oops, the 8x10 mat is not really 8x10!!!

The summary answer to your querry Nikonian72 is "because it doesn't."

OOPS: Now I understand FAQ in UHH is a reference category which is an IS definition. Thank you for setting me straight.. seems as tho a number of us fell for the why .... perhaps stating "We have added another FAQ to the FAQ list." would have avoided confusion. Excuse my ignorance of the meaning of the starting post.

Yes Linda, I indeed read the posting reference. And thank you for the work and explanation you did in the posting, many do not understand the basics ....

The referenced post answered what is, and not why. After 42+ years in R&D oriented chemistry/engineering/environmental work, IS and WHY are often two very differing questions.

Why is it every time you post , its like trying to read a leagle document . Were you living your child hood with a dictionary in one hand . You must have been a fun kid to play with . Did you get any other gifts as a kid besides word games . Your posts are like trying to read shakspear.

WHY is philosophical often, too often, resulting from starting choices and evolution after that. Is is the end result. There is an old statement "What is, is." often once the die is cast the expense of changing to something more ideal is cost/benefit prohibitive. The referenced article is fully understood and well written and documented.

Out of interest regarding sensors you may want to see Clarkvision.com reference article: Digital Camera Sensor Performance Summary (warning 64 pages)