On my opinion the only tanks that I feel are worth playing are Cho'Gath, Sion, and Dr. Mundo
Dr. Mundo is not a tank per se but I think he is worth mentioning
That said, Cho'Gath is actually underrated in this meta. He can sustain fairly well against a lot of matchups, and his ult allows him to scale into a tanky monster. Also, ADCs like Vayne and Kog'Maw are weak at the moment, so the ADCs that can shred him late game regardless of his HP aren't there to do this job.
Sion is also on the same boat, except that he trades early game sustain for having a lot of early game damage. He can dominate some matchups due to his immense early game damage. His damage will fall off late game, but at this point, he becomes a tank for his team, and like Cho'Gath, he can get a lot of HP (with the passive on his W).
Both Cho'Gath and Sion have one thing in common; they can potentially stack HP ad infinitum. Having a lot of HP is actually needed in this meta to survive the amount of true damage and burst we have. Most other tanks feel weak and not that much tanky, since they can't get as much HP as Cho'Gath and Sion. Because of this, true damage and armor shred effects tend to be a lot more noticeable on them.
Dr. Mundo doesn't get a lot of HP, but his ult allows him to sustain a lot. So much that he becomes very resillent to resistances shredding and true damage. Plus, like Cho'Gath, he can sustain fairly well in the early game. He may not get as much tankyness as Sion or Cho, but he compensates for that by having a lot of recovery, which can also allow him to survive.
Other tanks simply don't have the stats needed to compete with the damage creep, and by the time they reach their maximum tankyness potential, they are going to be shredded and will still die in seconds. They don't even have the sustain needed to compensate. The same isn't true for Cho'Gath and Sion, because they can actually become tankier than what their base stats and the items allow them to. I mean, how often can you even get 4000 HP on a champion not named Sion or Cho'Gath, outside of extreme late game? And how often can you get more than that?
Maybe it is time for Riot to look at tank items? I mean, tank items scale fairly poorly compared to damage items. I know that tank items tend to be cheaper, so that you have an option to build when you fall behind against an enemy champion that builds damage. But I feel that, in a meta like this, that doesn't matter much; even if you build a bit tanky, you are still behind, and the enemy still dominates you. You are still going to die, even if you may take a bit more time to.
TL;DR: Tanks are weak because they can't get the stats needed to survive against the damage creep and anti-tank tools we have in this game. Cho'Gath, Sion, and Dr. Mundo are the exception, since their kits allow them to either become tankier than other tanks, or have the sustain needed to survive.