@dokidoki - I think the reason we don't see it often, is because adventurers' guild acts as a
guild. That is, they hold monopoly on serious adventuring activity, and if some rogues go around pretending to be high-ranking adventurers, they deal with them. Possibly lethally.
There is a reason why there is only one Adventurers' Guild in the whole world - they got rid of all the competition.
@shanoc5902 - indeed, that's strange. Usually, feudal lords and other rulers pay for monster extermination commissions out of taxes. Because they want the taxes to keep coming, instead of ruling a monster-infested wasteland.
@seanaron - you can just re-read the work's name. He was a "hero" (=glorified assassin) and a "sage" (=archmage). In short, he has all the power in the world.
@trenslime - on occasions like this chapter, MC seems bad at dealing with people, but if you look at him overall, he is also quite good at getting along with people and flattering them. You shouldn't take anything he says at face value.