There are two situations where tanks are strong; tank items/scaling are actually strong, and damage champions are strong.
The former situation happened a few times, and tanks could just run anywhere. You would easily get three tanks on a team, and they would just have their way with any team that couldn't kill them. This is now how things are right now.
The latter situation is when high damage champions are strong, and tanks are required in order to actually deal with the damage. In this situation tanks are not actually strong, their utility is simply a requirement. One of the key differences between this situation and the former situation is how much tanks invest into damage, because if they need to focus on damage to do their job it means tank stats are actually weak.
To put it simply tanks are not actually strong in the overall game, they are simply doing what they must to get by in a meta that won't let them sit there and do their jobs in peace.