Plants do fight back, but the fight happens in S L O W motion.
Plants have evolved toxins. It’s not a coincidence that medicines and poisons come from plants. Or that the plants that we like to eat were selected over eons for their less bitter taste until—nearly defenseless—they could not survive on their own. That type of care and human selection—vs. natural selection—is called agriculture.
Some insects are specialized eaters. They’ve evolved resistance to select plant toxins and thrive by eating what would kill other creatures.
Monarchs are so specialized that the only food a monarch caterpillar can eat is milkweed.