I am Sandy Petersen, author of Call of Cthulhu and co-author of the later editions of RuneQuest. To be fair, Call of Cthulhu is, obviously, a really good target for "theater kid" stuff. Angsty 1920s jazz? Oh yes. Better for their ideas than D&D, frankly. And RuneQuest, developed around a world literally designed by an old-school hippy, is ripe for their meddling. And @ratbitebaby is 100% wrong.
I started roleplaying in 1974. My first game group (lasted til 1980) were all from the local university, which had and has a large theater production group (BYU).
I can't speak for other groups, but ZERO of us were what you'd call "theater kids". One of us got a job as a NATO janitor(!); another worked in an alloy manufacturing plant, yet another was an avid deer hunter, a fourth was a lawyer, and so forth.
When I started playing with "outside" groups mostly from California in the early 1980s, we also only rarely met "theater kids". By 1985, they did start creeping into my games. I don't know how many were in D&D by then. But they were not there at the start, and having met Gary Gygax, Jim Ward, Dave Arneson, and Dave Hargrave personally I can say they were the complete Platonic ideals of Not Theater Kids.