Because “kochen” isn’t a good translation for the english word “to cook” - at least not in all scenarios. When used with an object (etwas kochen, like Wäsche) it means more like “to boil” or “to brew”. So you “kocht Kaffee” (but one never “cooks coffee”). And it certainly is not true that you “ein Steak kocht”! So there’s an overlap in meaning but it’s not the exact same thing.
This makes kochen a partial false-friend!
I’d call the mode on the machine “boiling wash” or “hot wash” (which is probably 60 degrees rather than 90). But from other comments I’m understanding that Germans separate laundry into “Kochwäsche” and some other part(s), and there is no general term for that because it’s not common.