Why would the Admirals rule, they're soliders
I don't know what manga you are reading, but the WG exists outside of Marines. Marines are commoners who want to fight evil, the Gorosei were born as gods to rule the world. Statements like 'errand boy' and whatnot really show how poorly you understand not only how this manga works but real life too probably. Soldiers are meant to do action. Rulers, well, rule. Lmao
When someone decides to revolt against a royal family for example, they send the military. This is not that complicated.