There is no evidence that they do.

You say this, but do you know anything about the research and science in this regard?

Here’s one intro clip from Wikipedia, there are also many thousands of scientific studies related to learning about how different organisms feel things if you want to learn:

Crustaceans fulfill several criteria proposed as indicating that non-human animals may experience pain. These fulfilled criteria include a suitable nervous system and sensory receptors; opioid receptors and reduced responses to noxious stimuli when given analgesics and local anaesthetics; physiological changes to noxious stimuli; displaying protective motor reactions; exhibiting avoidance learning; and making trade-offs between noxious stimulus avoidance and other motivational requirements.

That is kinda what I was hinting at. If we define pain as something that causes a defensive (or similar) reaction, then sure: Plants react to “pain” in their own way. I have never seen a jellyfish react to “pain” though.

When my chinchilla starts barking, it’s easy to assume that he is in pain or otherwise uncomfortable, but to say that he is truly in pain is impossible. However, when animals hurt their foot and start limping, it’s a good theory that they are reacting to pain. My examples only include mammals though, to your point.

Excuse my rambling. I am not disagreeing with you, but just thinking out loud.