Only explanation: No physicists were involved in painting these cows. (Everybody knows that physicists only know spherical cows) Kind regards, Roel (yes, physicist by origin)

Physicists switch between square, spherical, and cylindrical cows depending on the coordinate system they are working in. However they would be squares with a unitless volume and length of 1. There is no need for rectangular cows, because you can just redefine the basis vectors to make it square.

Can you make a square cow from a spherical cow using only compass and straight edge?

I came here to make this same joke good sir! You beat me to it!