Was it because a “rubber” also meant a “condom”?
I’ve never known the eraser on the end of a pencil to be called a “rubber” in the US. The use of rubber to mean condom in the US dates from the 30s, so that predates my memory. Though we do call the rubber devices we put over our shoes rubbers which leads to some funny comments. In any case, I thought that all of these were called that because they were either made of some sort of rubber or rubberoid substance, rather than “rub out.”
edit: I see that I have it exactly backwards. The plant is so-named because it yields a substance that “rubs.”
edit14: somehow one of my many edits of this note got posted as a separate note.