I think a big reason behind the tank meta is how cheap Frozen Heart is
There's no comparison between tank and fighter items really
While a damage dealer might have a phage and a sheen the tank already has a frozen heart, can probably 1v1 the damage dealer because usually tanks have high base damages, as well as being way better in mid game teamfights
It used to be over 3000 gold by the way, Riot kept reducing the price for some reason and now here we are.
The item is 140% gold efficient without even counting the AOE AS slow, that is insanely overpowered