رَاعٍ comes from the idea of tending or caring for something. A person entrusted with people's rights, property, education, or safety receives not only authority but also a trust.
A teacher is answerable not just for finishing a lesson but also for fairness toward learners. A shop supervisor must protect entrusted property and maintain accurate accounts, not simply collect payments.
مَسْئُولٌ is related to the root س أ ل, “to ask.” The word itself evokes being questioned about one's duties.