The idea that you can cross the same type of creature all across a campaign, all the way from level 2 to 20, and it keeps appearing buffed to match your party is garbage in terms of immersion and mechanics.
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.
Hell, we even have monster races in 5e - there can be a goblin/kobold/etc PC that is way beyond the level of any Monster Manual counterpart.
Why can't the same apply to monsters?
