Where did people get the idea that the most prominent media franchises derive from adult literature?

http://forums.comicbookresources.com...d.php?t=391845
Even back in the 1980's, Star Wars and other PG/PG-13 franchises had higher box office than other franchises.

Other than Shaft (six prose novels, four films), I can think of few prolific R-rated adventure film series derived from prolific novel series.