Some single-player adventures are very strong and require good skills... Although I agree that it's not always possible to level up, etc. I didn't attribute such mystical power to them. I never level it up myself ;) Unless it's in a campaign.
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal