I would have agreed with you before they pointed out that "frozen water" gets a word: ice. Honestly, I think it's reasonable: people deal with frozen water far more than they do boiling water, but it changes it from a case of "what are they talking about?" to "okay, where do we draw the line?" for me.
Ice, slush, sleet, snow, graupel, hail... And within there is a subtype "black ice", a compound noun that isn't really just a description (it's not black, it's nearly invisible - a similar sense as another one, "black hole", which you'd never figure out from the components alone).
We have a lot of words for "frozen water" because it takes a lot of forms. As far as I know "boiling water" is only one thing so we've never needed additional words to distinguish it.
But water that has boiled into gas also gets a word: steam.
As far as I'm aware, there is no separate word for freezing water -- i.e. water that is very cold and will, if it continues to get colder (and has something to crystallise around), turn into ice.
So the symmetry seems complete: ice -> freezing water -> water -> boiling water -> steam.
Steam?
Well, being pedantic, my favorite hobby:
Frozen water represents a state change and that different state commonly gets its own word: ice/water/steam equates to solid/liquid/gas
Boiling/freezing water represents the state of the liquid, not the transition. Its descriptive. Water boils away into steam, or freezes into ice.
Should we consider luke-warm water also singular? What about body-temperature water? cool water? It makes sense not to treat adjectives/descriptive words combined with the subject as singular because the definition already exists in the root of the words (meaning of adjective word + meaning of subject word). Blue clay is another example, why would that be a singular?
It really only makes sense to me in the rare cases where the combination words represent something different or non obvious than the combined meanings of the two words (i.e to 'give up')