In a post from December 18, 2024, I posed the question, "Why are paintings—in general—rectangular?" Images placed in human dwellings follow their structure. In caves, we had irregular "pictures" incorporated into the rocky environment. Later, in proto-houses, huts, mansions, and castles, up to modern housing, it's always the same: A FLAT, PROTECTED FLOOR! We need a flat floor (or second horizontal plane) that is parallel to the horizontal plane our eyes perceive, dividing the passage in two: what is above and what is below. Remember that when walking, humans essentially enter a process of losing balance (potential fall) and regaining balance; each step is a balance-disequilibrium. 

Therefore, the traditional painting (and specifically its frame) follows this principle, being parallel to the ground, due to a need for psychological balance, in this case.