Spellcasters get to unleash their full fury on the world, a very limited number of times per day,
You do realize that is only true for the first third of the game, right?
mostly at a safe distance so they can protect their anemic hitpoints.
Mr. Druid would like to have a word with you, as would his pet bear.
Their spells are super powerful but they are balanced by their spells/day, and the necessity to use some of their magic for purely defensive purposes.
Meanwhile, melee characters run off of hit points, which tend to go out the door really fast unless healing magic is involved. They're not exactly eternal engines of war.
Throw spellcasters through a 10 encounter a day campaign, they'll come out pretty beat. Throw melee through one? They'll come out just as bad, because they can't recharge their HP. This is a problem for people on the front line.
Melee types get to unleash their full fury in the form of full attacks. They can do it an unlimited number of times per day, but the balancing factor is they have to trade blows in order to do it. They can close with the bad guys and strike first(once) before they get full attacked on the monsters turn. Or they can wait and get smacked first by the monster(once) then get to use the first full attack on their turn.
Why is this better for balance than having everyone be able to full attack all of the time?
Pounce essentially gives any melee type the ability 'All Full attack, all the time'.
Incidentally, this works only if you're not on terrain that prevents charging. Which isn't exactly uncommon or unreasonable to encounter in games.
Notice how warlocks and rogues scale at half that rate? It breaks the intended mechanic of the game to deal out huge damage, unlimited times, with no cost involved.
But you can't charge all the time, so I guess it's ok.
My opinion may be flavored by the multitude of 2-h power attacking goliath barbarians and warforged FotF monks and shapechanged buffed pouncing druids doing 200+ damage a round that I have played with.
From my experience, Pounce isn't game breaking because having a large amount of full attacks is not what breaks the game. Hell, at the level the Druid gets Shapechange, +200 damage is trivial.
Planar binding nightmares, imps, genies, and Solars breaks the game. Using Magic Jar to take over a Balor to free yourself of your feeble meatbag body breaks the game. Gating in Epic monsters breaks the game. Shapechanging into Chokers or Chronotryn breaks the game.
Pouncing is downright tame by comparison.
Of course, this may be colored by my experiences playing full spellcasters.