Language develops, with words added to the language, as the need to convey more specific information occurs. You might just as well ask, why doesn't the language of natives on a tropical island in the South Pacific have a word for 'snow'.
In fact, we do have antonyms for prepositions:
on / off
inside / outside
over / under :: synonyms : above / below
on top of / underneath
synonyms for conjunctions:
as / since
because / for
You meant 'antonym;, not 'acronym'; and I wonder if you really meant 'interjections'