I guess it's the stereotypical geek absolutist empirical mindset at work. Some people just need everything to fit neatly into an absolute structure. I'm often guilty of it myself.
No, what it is, is that levelling up and increasing all those numbers is a reward mechanic *because* it represents greater power in the game-world. Without that it's pointless for me. I would not be interested in playing D&D in a game where the GM levelled up everybody in tandem around my PC.
There are perfectly good games out there where PC power does not increase exponentially in relation to the world, like Runequest. If I want a game where after a year's play the pirates are still nearly as much of a threat, I'll play a game like that, not one where I have 20 times the hit points yet in practice everything stays the same. To me that's extremely unattractive game design.