One I'm actually sort of surprised nobody has brought up (and one which may generate some backlash)
The player base, by and large, does not want a perfectly balanced game.
3.5 was unbalanced; 3.5 was also very successful. 4e fixed most of the balance issues--and was not nearly as successful.
The history of D&D, I would argue, is one in which one-upmanship and "I have something you don't have!" play a critical role. Many players don't just want to have powerful characters who can do cool things--they want to have powerful characters who can do cool things that the other characters can't do.
In fact, I would go so far as to say that "I have something you don't have!" is one of the main pistons driving the engines of gaming in general.
Why was Magic: the Gathering so wildly successful? Because players bought cards by the case, hoping for that elusive rare that would give them an edge. "I have something you don't have."
Why do people pay thousands of dollars of real-world money for a trading card that will give them an in-game World of Warcraft mount that is only cosmetically different from the mounts they already have? "I have something you don't have."
Why have some of us devoted YEARS to manipulating the rules of this game, bought (in some cases) upwards of 50 rulebooks for a single edition, and pored through them for that single feat or spell that would unleash some devastating new synergy?
Again, on some level, I would argue that it's "I have something you don't have." And I think the fact that 4e tried to do away with it by establishing perfect parity of character capabilities at all levels is one of the major reasons it failed to resonate with a large portion of the playerbase, and why Pathfinder has thrived despite being at least as unbalanced as 3.5.