It doesn't help riot nerfed core tank items a few patches back, and still hasn't addressed tanks being unable to tank because of their oversights. So I'll outline some details for others not in know.
Randuin's omen, with armor and health, it was a prime choice against most assassins and adcs, but that was when it reduced the (basic) attacker's speed by 10% and attack speed by 20%. Which helped against quick and kiting enemies such as ashe, lucian, and kalista. They said this was too much (despite that popular champs were unaffected b/c of built in mobility, such as lucian, and caitlyn) and removed the auto attacker slow component, as well as reduced the basic attack penalty to 15%. So no more outrunning the one trying to kill you. And while 15% sounds like a decent penalty, I'll remind you that the most popular items for AD champions have more than 30% attack speed boost, such as the phantom dancer at 50%. Yeah, so that passive anti-AD is next to useless because its unnoticeable.
Banshee's veil, notorious for negating the first enemy ability to hit you. While crippling against single poke champs like nunu or malphite, useless against most mages and carries which have several pokes (or low cooldown pokes) such as vayne or nidalee. It also had a 25 second cooldown, which made it available for each teamfight, but taking any damage from champions reset this CD, which felt balanced. Again riot noticed the abusability of entire teams building this item, and instead of adding a disadvantage to multiple people holding it like the aura items, they instead increased this CD to 40 seconds. Though not quite doubling the time it takes to refresh, I'll also point out that the respawn timer for dying at the half hour mark (which most games end around) is 30 seconds. Which means you'll almost always respawn before your shield does, if if they are in your base, you'll need that shield to push them back.
Frozen heart, unlike the other two, had it's passive as an aura. Reducing nearby enemy attack speed by 20%, that's what it used to be, again they claimed it too strong and lowered this penalty to 15%. Remember that phantom dancer with +50% a.s.? That alone would cancel out this and the omen's penalties as they used to be, now they feel next to useless on most tanks and with such high costs.
Last whisper, offering a bit of attack damage, and a passive armor shred for it's wielder by 40% (the void staff is the magic equivalent), and with the right mysteries and runes, around 50%. Doesn't seem like such a good idea to spend nearly 3K on defense when you only benefit from half of it. What about health? Sure there's plenty of health items, but, blade of the ruined king (and liandry's torment the magic similarity) deal bonus damage based on the target's health. Did I forget to mention ignite? It deals a guaranteed 500 true damage (over 5 seconds), and you only get half healing effects during the time it's dealing this damage. There's also several antitank champions played fairly commonly; irelia & yi (autoattack true damage), zac & amumu (area damage scaling with enemy health), jarvan & xinzhao (armor shred), fizz & miss fortune & tristana (healing negation).
Tanks were useful back in season 1 & 2, but with defencive items being nerfed, high dps and burst champions have steamrolled. You may be thinking 'but what about crowd control abilities?' however you can't use them if you're dead. What's the point of CC if you don't live long enough to use it? A tank should be able to take more than 1 extra hit compared to the mage or carry, riot has forgotten that key detail.
Ahem 
As long as these items are as strong as they are right now, tanks will be rendered useless in the game. If you build tank, you deal less damage, but with these items your tank stats get shredded, all you do is use your crowdcontrol and then die.
