That reason in part is paranoia about "fat" in food which was conveniently used to distract from sugar and high-fructose corn syrup.
The cooking process it also far better, with the whole pan being uniformly hot and staying that way.
Eww, gross.
This is not necessary, you can (and should) actually clean your cast iron pans. I certainly clean mine.
Or clean as in industrial degreaser or varnish stripper?
Soap artist and still do that process though. Handmade soap will probably have some sodium hydroxide in it.
(Does anyone use that for dishes?)