1. The plastic bag smells of food. Because cats have a very sensitive sense of smell, they can smell very faint food odors, so cats tend to lick plastic bags that have contained food. 
2. Deficiency of trace elements: If a cat frequently eats plastic bags, grass, or even chews on wall plaster, it is very likely that it is deficient in a certain trace element, which may have led to pica.
3. Some cats like the sound of plastic bags. They enjoy the sound of plastic bags rubbing together, so they treat plastic bags as toys and often lick them or jump on them, trying to make them make a crisper sound.
