This wasn't necessarily true in the 70s and 80s. Since many games contained characters that were just a red pixel, with a black pixel thrown on to represent a “gun,” the covers were forced to incentivize people to buy something they might not purchase if they just saw the gameplay. So ink-and-pen art was commonly used, and real-life models, and, of course, chicks with really big boobs.
An ingenious Redditor—we think—is the first person to take those old concepts and apply it to games today. The results include a Skate 3 cover that I want a poster of now.
See more here.