Because it’s a ‘namul’ dish (implying it’s meant to remain)
Explanation&Learning Korean
This joke is a play on words involving the Korean term “나물” (namul), which refers to seasoned vegetables often used in bibimbap. The joke is that “나물” sounds similar to “남을” (nam-eul), meaning ‘to remain’ or ‘leftover’ in Korean. So, the punchline humorously suggests that because bibimbap is a ‘namul’ dish, it’s naturally meant to have leftovers, playing with the double meaning of the word.