We find that in fact reciting Shemona Esreh of maariv is optional but rishonim and achronim describe it as obligatory. Well, if it has attained the standard of an obligation, then certainly chazaras hashatz should be required. It cannot be both obligatory and optional at the same time, especially in the days before siddurim existed. But if despite its categorization as obligatory as mincha and shacharis there was never chazara shashatz, EVEN BEFORE there were siddurim, then certainly now when we DO have siddurim chazaras hashatz should be unnecessary even for mincha and shacharis.
And if it really is optional, then we have no obligation to recite it at all.
You are right in a way- if it is now considered an obligation, why isn’t there chazoras hashatz? The Eliyahu Rabba, (also brought in the Mishna Berura 137-1), answers, that although we accepted to daven maariv as if it is an obligation, however in essence it is still a minhag and it isn’t enough to me “matriarch” the tzibbur, and say chazoras hashatz.