Why are bread pans rectangular?
I don't think there is a baking reason behind the design.  Here is a loaf pan from 1897, for example.  Perhaps the popularity of the rectangular loaf can be traced back to the Pullman Palace Car Company in 1868. In the same article the author points out that early 18th century European bakers were using square tin pans to create loaves with minimal crust. I would suggest that this was more of a style preference, as opposed to physics.