I am pretty sure you would be ok fighting humanoid NPC both at level 2 and level 20. Why aren't you ok fighting monsters that scale.
Because playable humanoid races are SUPPOSED to cover the entire range of power in the scale, from the common villager to the legendary hero or the ascending demigod, while monsters are supposed to have their own consistent level of threat and that consistency is important for the believability of the setting.
Also, there's a reason if the thread opens mentioning the bad examples of one... well ,no, actually TWO under-leveled archdruids: because consistence between what you tell to the player and what you put in front of them is important even for humanoids.
Don't bring the Big Bad and Dark Lord down to level 3 so my party can face him early in the story. Give me the brigand leader as a villain appropriate to my presumed level of competence and leave the Dark Lord for when I'll be badass enough.
...Which are all things we already discussed, by the way. But since the same objections keep being made I think it's my right to repeat myself to answer.
