As discussed in answers to this related question, we repeat the amida of shacharis and mincha to aid those who do not have a siddur, have not memorized the brachot, or who cannot read, but we don't do so at maariv because maariv is a tefillas reshus ("optional" prayer).
However, the needy davener just described seems to present a need for saying the amida out loud. There must be subtle gradations to bracha levatala. We pray the maariv amida despite it being reshus, but we do not do so out loud to help those who cannot daven on their own. What should the needy davener do? Not pray? Why do we not say the amida out loud for such people?