The parasytes cannot eat plants.

Watch Migi recoil from vegetation in Episode 4, watch Shinichi explain this limitation as though it's a law of physics — yet it isn't in Hitoshi Iwaaki's manga.

The anime production staff, writing in 1988 and 1989 during Japan's peak bubble economy, introduced this constraint as a biological inevitability, a fact of parasitic nature. They made starvation geometric — the parasytes are not failing to access food, they are neurologically repulsed by entire categories of it. This is the language of anxiety spoken through anatomy.

Japan in the late 1980s was purchasing Manhattan real estate and Picassos, yet the nation's resource vulnerabilities were undeniable: ninety percent of oil imported, agricultural land shrinking, arable soil finite. The bubble was real money on fake foundation. The anime's parasytes move through Japanese cities consuming the only renewable resource available — human flesh — because the writers understood at a cellular level that abundance was not stable, and they could not imagine alternatives.

Migi doesn't refuse plants because parasites are strictly carnivorous; Migi refuses plants because in the collective unconscious of late-80s Tokyo, depending on what you could grow was a luxury the nation could no longer afford. The monsters are not aliens — they are Japan looking in the mirror, seeing a creature locked into a single food chain, and recognizing that chain was about to break.