First, they're considered a low-grade food. The Chinese word for vegetable has negative connotations in modern slang: ts'ai (vegetable) or ts'ai-niao (vegetable bird) describes a person who is stupid or boorish.
一般人多半是把它當做下等的食物，正如現代流行語，形容某人「很菜」、「菜鳥」，表示他既笨又土，具有價值否定的意思，所以古代祭祀是不用蔬菜的，「禮記」就說：「蔬食不足祭也。」