There is also no fluff saying that the mass of humanity(or demihumanity) consists of 0-level and 1st level losers.
In 4e, this is exactly correct! In 1e, 2e and 3e however, there was indeed that fluff - demographic info - putting almost all NPCs at 0th/1st. 4e does have its 'human rabble', but these can be used for untrained peasants. You certainly can run a 4e campaign where thousands of 7th level cavaliers clash against battalions of 3rd level guards, with 4th level wizards here and there, and that is entirely RAW. In 1e-3e it would be a significant deviation from RAW. 2e: "There are no armies of 4th level Fighters". 3e: "The great majority of soldiers are 1st level commoners or warriors".