Because pigs are smart, they can't think and think about animals without brains, and people are the tallest animals. This is millions of years of evolution. I can't describe how embarrassing it is for a person to say "Wow, you are as smart as a pig" or "you are as smart as a pig". In addition, pigs are domestic animals and the main source of meat we need. Because most of the pigs you see are degenerate domestic pigs, their usual habits are eating, drinking, sleeping and growing meat, which naturally makes them feel stupid.