Cover art is designed with visual appeal in mind, and the best cover art grab the buyer’s attention from across the aisle, begging them to take a closer look.
Video games used to rely on cover art to help sell cartridges- they presented polished and generally exciting scenes to lure gamers into buying the game, then gamers would discover the in-game graphics looked nothing like the cover art.
That’s why it’s best to think of cover art as a piece of concept art, rather than a faithful representation of what to expect from the game.
With that in mind you should check out GeekTyrant's collection of 15 Best Pieces Of 1980s NES Cover Art, where each piece is paired with a video showing the actual in-game graphics, so you can get a feel for how far off the cover art really is!