thats simply not true.
Tank ekko does no more damage than any other tank, his base damage are more on the low side actually, even more so with all the nerfs.
The thing is that tank ekko has utillity and tankiness close to other tanks(still a bit less on both), but unlike them he has the mobillity of an assasin, which allows him to deliver his cc and damage on the right targets whil he's exeptionall hard to kill, cause he's tanky enough to survive a lockdown(unlike the assasin version) and after that can easily escape with his mobillity.
The main strength of tank ekko is his snowballing, as you can't escape him, you can't turn around on him if he's ahead, and you can't shut him because of his high mobillity+tanky built.
Similar with tanky fizz, thoguh he has more damage and less utillity, but the combination of tankiness and slipperiness makes it hard to deal with him.
Again, compared to tanks like mao or Seju, the base damage of eklko and co is nothing exeptional, and compared to fighters like irelia or jax it's rather low, same for base tankiness. The main issue is how much safety the combination of tankiness and mobillity offers, and how hard it is to peel against them as landing a single hard cc, which would cause there assasin version to die, isn't nenough to stop them.
Another issue with fizz and ekko is that the omprtant parts of their kits are hard to avoid, especially with a tank built.
Defensive mobillity is just very strong and frustraiting. If you look at all the champs that usually are complaint about: Lee, Fizz, Ekko, LB, Riven, Zed, Lucian, Ezreal, Kindret... they all have plenty of defensive mobillity.