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!