No one realize this probably because anyone that just leeching probably brought up so many soldier or mercenary to the point where exp they got get divided so much until reach 1 exp/monster. imagine if an adult already have lv 3 because doing simple task everyday they would need 400 exp/400 monster to kill to leveled up so no wonder if no one even thougt it was possible. And it worth mention that MC has detection skills to know the effective range and easily kill many monster alone.
But i doubt no one know about this loophole. Considering that knight have high level they probably making it a secret.