I can see your point, particularly with the heartichoke + flytrap (it's insane if you let the plant activate that combo)
But plants are not stronger than Zombies. I also play both and I can tell Zombies are equally annoying with right sets of card.
Brainy classes with medulla nebula can make any plants cry in frustration, not to mention when you face grave zombies, they give you headache and nightmares. I myself hate facing sports decks, as they are so tough and have many removal / healing cards, plus the annoying Warlock and going viral combo.
Plants you used as example of OP in fact have low attk and health, so they can be killed quite easily before being buffed or protected. I think plants become strong later in one match, but if you overwhelmed them from the beginning, than Zombie is hands down able to destroy them.