I think the issue mainly is you are comparing apples to oranges.
Assassins jobs are to kill you instantly. Which in and of itself is toxic gameplay since it basically removes counterplay other than don't let them near you. Its kinda hard to stop them when they can unleash their entire kit in not even a second and you are dead. And I am not talking a fed assassin. I mean even 0/4 with 50 farm 15-20 minutes into the game Zed/any other relevant assassin atm that makes me always wary once they hits 6 due to their nature.
Tanks aren't killing you fast. And Bruisers which people say are tanks anyways aren't killing you fast. Best example I can think of is a fed Irelia or Darius/Garen, they aren't killing you instantly. When fed you got several seconds to think of something to do or have teammates react.
It might be seen as double standards but the way I see it. You can itemize and fight tanks much easier than assassins. Assassins can kill you faster than you can even react which is the big thing people hate. Only reason they exist is basically to counter ADCS which would be unstoppable without them. But still doesn't change that its still bad gameplay.