Tanks are basically generalists, much like the Support roster. No one Tank is countering any specific Tank, but the ability of certain Tanks to… harmonize with their team to bring a bigger challenge against the enemy Tank or team is creating this perception.
Overwatch has never really been a series of counters or “paper-rock-scissors” elements, but rather a series of match-ups with different advantages or disadvantages. Bringing the right team setup can create an insurmountable wall of advantage that can be difficult for any one character, or Tank to pierce through.
They were not. Game would not be in a static 3 Tank, 3 Support composition that lasted years if they were.
Even by the end of Overwatch 1, the game had a fairly static composition predominately composed of Rein, Zarya and Ana. Tanks weren’t weak in Overwatch 1, at no point were they ever considered weak. That’s why Overwatch 2 moved to 5v5 and eliminated one tank from the roster.
Even common Tank complaints about being crowd controlled endlessly in OW1 was the result of other Tanks and their ability to control. Tanks in Overwatch 1 were built with the idea of going against enemy teams of non-Tanks; they weren’t really designed to go up against other Tanks.
Brigitte was the catalyst, but triple-tank was a thing before Brigitte became playable. And after significant tweaks to Brigitte, she was abandoned for another Support even before Baptiste became playable or inspired the "bunker’ metagame.
Because the Damage-role doesn’t really counter the Tank or Support role. The original design for Overwatch was Offense vs. Defense. Heroes like Torbjorn had advantages against heroes like Cassidy. Junkrat advantages over Tracer. Mei could win every single CQC 1v1.
Most of those tools are still in the game, and still touting the same power levels they’ve always had. But every Tank is just as capable of crushing down those Offense and Defense heroes; and every Support is still capable of denying them.