That's because early explosive bombs did physically resemble the popular image of the bomb. Just take a look at this illustration of a fragmentation bomb from the Huolongjing, a text from the Chinese Ming dynasty. It shares the same shape as drawings of bombs commonly seen in popular culture. Primitive gunpowder bombs first appeared during the Song dynasty. Soldiers lit the fuse of round cast iron or ceramic shells filled with gunpowder, which were then fired using catapults. For more on the early history of bombs, I would recommend reading Military Technology: The Gunpowder Epic, volume 5 part 7 of Needham's Science and Civilization in China, published in 1987.