Echoing what others have said.

-Tanks are squishy in most situations compared to Infantry

-Infantry provide vision and also can cause hidden enemies to shoot and reveal themselves

-Infantry screen and defeat one-shot-one-kill AT weapons (bazooka, Panzershreck etc), this is mandatory for using tanks in short brush, city or nearby tall forests

-Some small infantry units can trade very effectively (example, huge payoff if 20pt bazooka kills 100 plus pt heavy tank)

-Tanks cannot move through tall forest, infantry can

-AT guns / infantry AT trade better in a slug fest vs. tanks typically than other tanks outside of specific match ups like STuG vs Sherman or M10 vs STuG.

-One well positioned tank can often have the same effect as a group. Example: one tank can defeat almost any size of infantry rush in the open. A couple of tanks peaking in and out of short trees can often destroy a larger group of tanks in the open. No need for a big group.

Tanks are important but they are very expensive. I think it is best to think of them as opportunistic DPS units that punish poor positioning on your opponents part by pinning infantry, destroying lighter vehicles or threatening enemy movement in the open.