Earlier quoted context omitted.
We have the word slush to mean a mixture of ice and water. A single word for boiling water would occupy a similar conceptual space. While these are not separate states of matter, they ARE special thermodynamic systems, with the particular property that they tend to remain exactly at the phase transition temperature while heat is added or removed from the system. This is a somewhere esoteric technical distinction, but…
but what about boiling milk? or boiling oil? I get your point, I just don't understand why we would have a word for boiling water but then still need boiling-x for everything else that boils. edit: In those other languages is it like how we use ice? where water is the default, but it could mean any frozen liquid?
Asking for the "reasons" behind a certain word existing is sort of like asking why the human body looks the way it does. Sure, scientists may have good theories why it was evolutionary advantageous to have five fingers and no tail, but in the end the only answer that's for certain is, "because it evolved that way". So the answer is, "we" have a word for boiling water because people found it useful to have such a word.