I thought normal people could also level up from killing things. The whole arc with his first party covered that. Everyone in the town didn't level up after a certain point because they just killed stuff near the town and lived their lives instead of thrill seeking trying to find stronger beasts. This plot line is strange. Also a plot hole earlier was that humans stop leveling up at a certain point because there aren't many stronger monsters and the strongest humans kill other humans to level up so they're obviously murderers. Uhh, did they forget that war exists?