Because if the main character has the same level cultivation as the enemy, he will one shot them.
If you guys read the novel, there is a part/arc (the manga skipped it completely) where the MC somehow is at the same level as the enemy of that arc and guess what? It is pure stomp He is literally casually soloing 3 enemies that have the same level as him.
At this moment, the MC is always below the enemy of the arc for 1 tier, but later on, the gap is starting become larger where the MC is facing an enemy that 2 or 3 levels ahead of him and it is 2 enemies at the same time. It is to balance out the fight because the MC tend to have too much OP treasures, usable items, and skills despite having a lower level than his enemies.
Another reason why MC can handle an enemy that stronger than him because he also cultivates the mental strength. It is like a character that has Lv5 physical attack and Lv5 magic attack vs the enemy that has Lv6 Physical attack and Lv1 Magic Attack.