Unfortunately, this is just the current play style right now. You see a lot of teams made up of 5 off tanks, or a combination of 4 tanks/off tanks with one good carry.
The reason is that, just as you noticed, tanks have become pretty much self-sufficient anymore. They can deal out obscene amounts of damage with straight tank builds.
Galio, even as a tank, hits reasonably hard, and is difficult to take down (though I feel he more threatening played as a caster).
Garen has always been able to put out crazy damage because of armor pen. No need to stack damage items on him, because then he just dies too easily. He's more scary simply by being able to stick around much longer.
And the list continues: Amumu, Mordekaiser (a solid off tank), even Vlad (not really a tank, but gets enough HP that he can last a good bit of time and be annoying), can all put down some reasonably high damage without necessarily building for it.
Amumu is probably the weakest of all these by himself. But there's a theme here: Amumu has been around for a long time now, since before the current game style. The others I've mentioned are all fairly new champions, and have managed to render some champs near useless solely by their existence.
For example, Teemo used to be seen in almost every game (talking a year ago, back in beta). Back then, teams usually had one solid tank, either two casters or two phys champs, and a solid support champ. A pretty 'classic' team make-up would have been Kayle, Ashe or Teemo, Annie, Zilean, Blitzcrank. Anyhow, Teemo has never been a great tank-breaker. His role was more to take out the other team's phys carry.
But what if Garen is their 'phys carry'? Teemo can't deal with that. Teemo is not a tankbreaker. Annie can get some very high burst damage, but many champs now have so much HP and MR that they laugh at her damage, even from her full combo. Not only do they laugh at it, but they walk right through it and kill annie while they're at it. This is also part of why Eve is useless at this point. Eve can't pop out and take down a tank. She's supposed to go in, take out the carry, and get out. She just can't actually burst enough damage to take out all the tanky champs out now.
(I think you get my point by now: I'm not a fan of where the game has gone :P)
As for the tanks having all the disables, that was kind of the point. Amumu, one of the strongest tanks in the game at the moment, is meant to bandage toss in, stun one, and then ult. This holds the team in place while amumu's team can come in a get some easy damage on the enemy. So the tanks are kinda meant to be the initiators, and what better way to initiate than to put out some AOE CC of some sort.
At least, that's the current status of the game.
For breaking tanks, your best bet at the moment is probably Kog'Maw. He has a good passive for it, and works well with a bloodrazor. You just have to get enough HP and defense on him that he can survive long enough to take down a tank (or two, three....)