Why do people complain about tanks?

In my experience i never seen a problem with tanks in particuliar, i believe they fulfill the role they are designed to accomplish, yet many people hate on them and believe they should be nerfed.

Tanks NEED to be all around stronger than other classes. They usualy lack mobility while also being melee, so they require to be tanky to survive being focused a few seconds while also providing either damage or CC. They NEED to be tanky AND have damage(like juggernauts) or CC. If they aren't tanky enough they'll get melt before doing anything, and if they don't have enough damage or CC, players can just ignore them in teamfights (Just like people used to ignore old Garen in teamfights since he used to deal close to no damage)

Nerf any aspect of their kit and they'll become irrelevant and unable to do their designed job.

Tanks are one of the pillar of the team, almost mandatory, yet they can't function properly without the help of their teammates.