Because you wouldn't have to wait for a human to write the engine and write the game

We generate 2d art, 3d models, some other in-world data, and soon animations at our game studio. The huge problems of generating pure pixels without a persistent game state behind it is illustrated well by the recent MinecraftAI craze (where it's a cool demo, but absolutely unplayable for anything other than novelty), and exacerbated on the dev side by missing out on tons of "free" stuff you get from existing engines that already do almost every kind of physics ops for you.