Classes and levels are artifacts from pen and paper RPG's that were created to simplify the real world mechanics. In reality, a person using a sword go from wildly swinging it around to more deliberate strokes, he would learn to parry, and when to thrust, slash, jab, etc. He'd learn how to handle different swords, etc. Also the more one is in combat, the greater one's pain threshold becomes. But all of this is too complicated of a system to model using calculators and die. So the designers developed levels as a way to mock out how humans over time can get stronger, smarter, more agile, etc. The classes system is meant to reflect the fact that we all have strengths and weaknesses. I am bad at poetry, but good at programming computers. But again too nuanced for PnP games. Computers could do all those calculations, but it would be expensive and people are accustomed to levels and classes.
My problem with the gear grind is that gear is the end, and not the means. Gear is really nothing but a soft leveling mechanism, that lets the developers build tougher bosses and gate content. I understand why they did it, but that doesn't mean I like it.