Depends on team and elo... tanks are still viable, they just aren't the most fun to play. Because playing Maokai or malphite, you are going to get screwed hard in lane 1v1, you are going to be useless if their jg camps you and you get no help, and you are going to be useless even if you won lane if your carries are behind and have no damage to compete with the other team.
But give me some carry jg, a mid that deals damage, and a normal (old school style) bot lane, and put someone like maokai in front of them to engage and peel for them vs a more meta enemy team and... the other team can't walk past maokai to blow up your teams squishies, they're probably too scared to be the frontline for their team anyways, and your team has 3+ people who can just focus their firepower on whomever you cc as the maokai and pop them. Do this one by one and kaboom, maokai op.
Like I said though... the drawbacks are many including requires teammates to be useful, requires teammates to coordinate, requires you to not get obliterated in lane even if camped, requires (kind of) teammates to not leave you out to dry and then flame you for having a bad game when you sacrificed everything for setting up their success in teamfights only to be camped in lane and never have team back you up in lane or teamfights. People don't like conceding their ability to win. They want to control their own destiny, and that's why you don't see as many pure tanks (unless they're broken like mundo right now) even if the meta were a tank meta. Just like you can see a tahm or braum support... or you can see a brand or lux support. Even if the braum or tahm is more useful for the team (often is), you typically see the brand or lux picked instead cuz... fun and carry potential as an individual.