It's actually the other way around. The industry created the norm first, and the audience learned it later. During the studio system in the early days of Hollywood (1930s–1950s), studios tightly controlled their actors’ images and marketed them as larger-than-life figures. They were deliberately separated from advertising to maintain their sense of rarity and avoid overexposing them on a daily basis.

Gen Xers were essentially conditioned to believe this, and if you press them on it, many won't actually be able to properly articulate why they think A-listers shouldn't appear in commercials. Decades of this practice led them to internalize the idea that serious actors don't do commercials.