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.
Who said that monsters are "supposed" to be consistent? Even the monster manual says that "you can do with these monsters what you will" - the stat blocks are clearly more of guidelines that set in stone truths.
I for one think not modifying monsters detracts from the believability - so the party's fighter can hit the gym hard enough that he can take on a Fire Giant one-on-one, but not a single Fire Giant can go above or below his stat block in the Monster Manual?
