Naming of hydroxides
A lot of chemical nomenclature is historic and not very systematic.

Common names for chemicals are often determined by history and don't always completely describe the detailed chemical structure or formula. The theory of chemical bonding and structure was not fully developed until the 20th century but we did a lot of chemistry before that. People got used to common names for things and those names persisted even when more systematic names were developed to introduce more clarity about the relationship between names and formula/structure.

For example, acetone is the commonest name for propan-2-one but the common name gives no indication at all about the chemical structure. Many organic compound have very unwieldy systematic names (reflecting the gargantuan complexity of the chemistry of carbon compounds). Fluoxetine (the active ingredient in prozac) is N-methyl-3-phenyl-3-[4-(trifluoromethyl)phenoxy]propan-1-amine and that is too big even to fit on the pill bottle and even the formula, C17H18F3NO, is a bit of a mouthful despite not giving away a lot of structural information about how the atoms are connected.

In the case of simple inorganic compounds like simple hydroxides the situation is similar. The names arose because they described the key chemical parts making up the compound, in this case a metal (as an ion) and a hydroxide ion (OH-). Rather than the unwieldy but more precise names describing the number of ether part of the compounds, chemists apply a little chemical knowledge and keep simpler names for many compounds. Most chemists know that alkali metals (Na, K etc.) form ions with a single positive charge and, therefore, if they form hydroxides they will only contain one hydroxide. Alkaline earth metals (Mg, Ca etc.) form ions with two positive charges and their compounds will contain two hydroxides. This is because all atoms want to reach the lowest possible energy state, which usually is the elecron structure of noble gases. Since most chemists know this, there is no point in using a more complex and longer name. This isn't always true: transition metals can form ions with several different "oxidation states" (which basically means the charge on the ion in simple ionic compounds: iron for example is commonly found both as Fe2+ and Fe3+) and, in these cases the name needs to contain extra information to help the chemist tell them apart.