Because word pain loses any meaning then. You can't prove that your toilet bowl doesn't feel pain when you violate its homeostasis by flushing.
Don't try to equate living beings with non-living things. Doesn't work.
My argument is that all living things can feel pain, regardless of how it manifests for them.
It is not an argument. It is a statement. OK. What about short DNA fragments in a solution of DNA replicase. They are built from DNA, they are replicating, in a few billions years they can evolve, they aren't much more complex than a toilet bowl, undergrad molecular biology student can create them by billions.