But just make yourself invisible and everything’s easy. I love that game.
The problem is you’re talking about a different type of game. Action games are games which rely on player skill. Fighting games, action-adventure games like Assassin’s Creed or Uncharted or Tomb Raider, etc.
Meanwhile, many RPGs are designed to focus on the character. It’s the character leveling up, not you. So it should be reflected in the character’s abilities/stats.
You have a spectrum, of course. Most RPGs nowadays are action RPGs, which have a stat basis but include an action element as well. I’m playing Nier Automata right now and it’s kind of an RPG, where your HP and attack/defense are determined by your level (same for the enemies), but because it’s an action RPG you can fight enemies stronger than you and win. I’ve fought enemies twice my level and won–it takes longer, and I have to be more careful, both because I do less damage and take more–but it’s still possible.
A system with only enemies of the same level removes that dynamic completely.