Because that's not what Le Gray did. He would take images of the same scene at different exposures. That's easy. Swapping a completely different sky from another image.... not so much. Someone with the talent of, say, Uelsmann could probably do it but it's not something that is generally considered doable with film. Very much more difficult with colour neg and slide film than b&w as well.
I think you should accept that Isaac is right. I experienced this swapping of skies in connection with a very old photograph I found in my late father's collection of photographic prints. The print was from the late 19th century photographer Frank Meadow Sutcliffe. I tried to buy a larger print from the Sutcliffe Gallery in the U.K. but was surprised to find that the image they were offering, clearly identifiable as the same shot because the foreground was identical, had a different sky.
I was surprised to learn that skilled photographers using old-fashioned darkroom techniques were able to swap skies in an image as far back as that.
Just to check that I haven't got this wrong, I did an internet search and came across the following article from Amateur Photographer:http://www.amateurphotographer.co.uk/how-to/icons-of-photography/534602/frank-meadow-sutcliffe-1853-1941-iconic-photographer
In it you will find the following comment:
"Image: ‘Dock End, Whitby’, 1880. Sky tones couldn’t be captured using the wet-plate process, so Sutcliffe expertly print in clouds from another negative"
Since I have noticed in another thread that you have begun appealing to God to confirm your views, perhaps you should pay attention to a particularly relevant prayer, known as the Serenity Prayer.
God, give me grace to accept with serenity
the things that cannot be changed,
Courage to change the things
which should be changed,
and the Wisdom to distinguish
the one from the other.