The reason I always heard was because some do it yourselfer would drain the the transmission, thinking it was the engine, add five fresh quarts of oil and proceed to burn up his transmission.
Back at Miramar, with had a Navy ET Chief that was far from a mechanic. One afternoon after work he showed up with his 67 Impala (1972 or so) wanting to change his own oil
I said, well, start by removing the plug on the bottom of the engine and let all the oil fall out.
So what he ended up with is
an empty transmission
a new oil filter
and five quarts of new engine oil mixed right in with 5 quarts of OLD engine oil