Question

Why is it impossible for carnivores to eat plants?

Answer 1

Because carnivores only eat meat.  Or as the dictionary says, "carnivores feed on flesh" Flesh is meat. Not plants.

If a carnivore did eat plants instead of meat, then they'd be an herbivore.
If a carnivore ate plants and meat, they'd be an omnivore.

Answer 2

because carnivores eat only meat