Just because you do not understand it does not mean it does not exist.
Pain is a human defined "experience" to specific stimuli.
You cannot state definitively that plants cannot experience something equivalent.

"Plants respond to herbivore attack through an intricate and dynamic defense system that includes structural barriers, toxic chemicals, and attraction of natural enemies of the target pests. " -- Nih.gov

So I mean they're just "defending" themselves for no reason, right?
https://bestlifeonline.com/how-do-plants-protect-themselves/