Because they're the same parts, painted twice.

Super Mario Bros. (Nintendo, 1985) had to fit every picture, every level and every sound onto a small cartridge. In Nintendo's own Iwata Asks interview (2010), programmer Toshihiko Nakago says: "We poured our strength into figuring out how to use a little data to represent a lot." Each picture was colored from a palette, and in Satoru Iwata's words "you could only use four colors for each one." Keep the shape, swap the four colors, and it becomes something else. Shigeru Miyamoto: "making the greenery and clouds out of the same parts."

It didn't stop there. "The castle at the start is small, and the one at the goal is big, but they're actually the same castle" (Nakago); "We took the top of the castle at the goal and used it at the start" (Takashi Tezuka). Koji Kondo: "I used the same sound for when he gets smaller and when he goes into pipes." And Iwata again: "In this day and age, if someone tells you to do something with 30 bytes, you can't do anything. But back then, we'd be like, 'That's ten blocks!'"

The world, the shapes, the colors and the square hero in this film are our own, built in code (the first test of the game moved "only a square shape", Tezuka). No Nintendo art is shown.

Which other game reuses its pictures in a way you never noticed?

Sources: Iwata Asks, Super Mario Bros. 25th Anniversary, Volume 5, "Original Super Mario Developers" (Nintendo, 2010).
Not affiliated with or endorsed by Nintendo.
Sounds: Apple iMovie sound effects.