Trainers in post-game areas may have higher levels but most of them would never get through the E4. The Elite Four have strong Pokémon and tactics while the trainers are the same mooks you've seen before, just with higher levels. It's the same deal with the early Gym Leaders. They far weaker than the E4 but when you get to them they are still a challenge. RPG levelling systems don't make perfect sense.
As has been mentioned, the E4 do have a stronger team on remathes anyway. And the Battle Frontier doesn't count since it is a different brand of high-level challenge.
The Elite Four are a big end-game challenge to prove the strength of your team. If you did retire them, how would you end the game?