There are a lot of factors everyone seems to be completely missing.
1) Tanks aren't "unbalanced" in real life. There are MANY things that can limit their effectiveness and counter them. For instance, levels can be designed so that there are dynamic, logical options for anti-tank tactics. This could be hindering line-of-sight through environmental design, providing objectives in locations where tanks aren't as maneuverable, man-made obstacles to slow down, trap, or prevent movement. Maybe even objectives/mechanics that revolve-around/allow the tank being used for things such as positional control instead of a mobile death fortress.
2) Realistic scenarios can exist where they haven't completely optimized the vehicle. Weaknesses in tanks don't need to reflect exactly what exists at this moment unless the game is trying to be a perfect simulator of a real-world time period.
Yeah, if you plop a tank into a circular arena it's going to be unbalanced, but that scenario shouldn't be happening in the first place.
I don't play FPS games very much, certainly not war shooters, but these are elemental game design concepts that can all absolutely be utilized to reflect true tank capabilities without creating design problems.